Step 1: Before You Crack It Open...

Updated 31/01/2012
Thanks to MrJentis for some more ideas.

Before you go ripping it open remember that you can reuse the whole computer and not just some of the components.

1.-Install Linux
Linux is an open source OS that you can install on your old computer and use it as a everyday computer, for those of you out there that are not geeks that just basically means its a OS like windows XP that people give away free that you can use. You could do endless things with linux and there is a distribution to suit every computer.

Ubuntu-
This is for some of the better spec computers out there. Its is the most like proper a OS and it is the most user friendly. It can easily do everyday tasks like Surfing and sending email's. I needs:
1 GHz x86 processor
1 Gb of system memory (RAM)
15 GB of hard-drive space
Graphics card and monitor capable of 1024 by 768
Either a Cd/Dvd-drive or a Usb socket (or both)
Internet access is helpful

Ubuntu's system requirements have escalated over the last few released but there is a alternative version based on Ubuntu called Xbuntu

500Mhz x86 Processor (At the very least!)
256 MiB of system memory (RAM)
2 GB of disk space
Graphics card and monitor capable of 800x600 resolution

Puppy Linux-
This is a slightly less user friendly version of linux. Is is for those less functional PC's with not as good a spec. It needs:

CPU : Pentium 166MMX
RAM : 64MB
CDROM : 20x and up

Damn Small Linux
OK if none of the above options have worked for you, you must really have a scrappy computer. It is very basic and is not user friendly. It requires
486dx or 100% compatible processor
16mb ram (24mb recommended)
dual-speed CDROM (quad-speed or better recommended) (thats 2x or 4x)
1.44mb floppy drive (for boot floppy, if needed)

There are loads more distrabutions try http://www.linux.org/

Now I have never done the following options before so I can't go into detail on how to do them but you can google them:

Turn it into a server (web, printer, file etc.)
Put a big hard driver in it and make it a jukebox
Put a TV tuner into it and a big hard drive and make it a PVR
Use it as a firewall
Use it as a stress reliever
Automate Your Home
Use it as a answering machine
Make the monitor into a aquarium
Make it into a rendering farm
Trade It in
Put it in your car
Give it away/Donate
There are some more in the comments!

Step 2: The CD-Drive

OK if none of those last options worked for you its now time to crack it open. So lets start with the simple components and work our way up. So the 1st component is the cd drive.

1.Make it into CD player.
Yes, this one is very simple. All you have to do is take the CD drive and the power supply out and connect them together like they were in the computer. If your PSU is the ATX standard to make the power supply work without the computer short one of the black wires and then green wire in the big motherboard connector. You can use a piece of wire or a paper clip or just chop them off and then join them. Now all you have to do is hook speakers or your hifi into the headphone socket on the front of the drive. Then press the play/fast forward on the front of it.

2.Make it into a laser pointer
If you have a DVD-RW and are not planning to use it in another computer you could turn it into a laser pointer. USE with caution

See HERE

3.Make it into a safe
This makes a great hiding place. What you do is you hollow out the drive and you can use it as a great hiding place! If you are going to attempt this and are smart enough and can keep the motor mechanism (ie open and close with the open/close switch) I would suggest cutting the two black ground wires off the power connector and putting them through a basic on/off keyswitch. Therefore if you do let slip about it they wont be able to open it with out the key.

See HERE

4.Make a Roulette Wheel
This is a brilliant idea. If you can never decided who is going to get the coffee or are just indecisive this is the gadget for you!

See HERE

Step 3: Floppy Drive

The humble floppy drive. Gone and almost forgotten. They are handy in even the latest computers for flashing BIOS's and other tasks. There are hundreds of uses for floppy disks, all you have to do is search Instructables for "floppy disk" but there aren't so many for the drive itself. If you have a MacIntosh floppy drive, your in luck as there is some uses for them.

1-The eject motor
The eject motor in apple floppy disk drives are very strong and efficiently would make a fine motor for use in a robot or another project.

See HERE

2. A Amp case
If by any chance you have a Apple PC 5.25 Floppy Drive and a Sonic Impact 5066 T-amp or similar you can make a cool case for it. If you were adventurous enough make it into a slot loading dvd drive.

See HERE

3.Airsoft Landmine
There isn't a guide for this but some people in the comments have mentioned make a mine of of it ie. when you step on it the disks shoots out. It could also be used a a normal handheld launcher. If you are interested tackle the people in the comments

Step 4: Hard Drive

OK, believe it or not you can do alot with a old hard drive. If you have gave your best efforts to try and fix it and its completely dead you can try some of these-

1. Make it into a clock-
Yes the disc out of a hard drive actually makes a pretty good clock and it looks cool.

See HERE

Or if you think your really good, try this one!

See HERE (If you dare)

2. Use it as a fake
Now you can impress your mates with your USB hard drive thats actually made out of memory sticks.

See HERE

3.Turn it into a rotary input device-
Thats basically a wheel that you turn that controls your computer

See HERE

4. Make a tesla turbine
A tesa turbine is a thing that converts air and water motion into eletricty.

See HERE

5.Make a really strong fridge magnet
In hard drives there are super strong magnets...These make great fridge magnets!

See HERE

6.Make it into a safe
This ones pretty simple; gut a hard drive and store stuff inside it

See HERE

7.Simply Use It as a Coaster
Self-explanatory. But the only thing I would say is that if you have a USB cup warmer you could take the heating plate out of it so it would keep your drink warm!

See HERE

Step 5: Power Supply

There arnt as many uses for the power supply but thats not to say the few there is for it isn't good.

1.Convert it to a Bench Top Power Supply
Computer PSU's provide stable voltage with short circuit and thermal protection. Makes it perfect for a bench top Supply.

See HERE for my very own Instructable on the matter

2.Build A Fireworks Launcher
DONT ATTEMPT THIS IF YOU NOT OVER 18 but if you want to risk blowing your head off be my guest.

See HERE

3.Recycle it into a fume extractor.
All you have to do is take the guts out of it and hook the fan up to 12v. Great for soldering!

See HERE

Step 6: Motherboard, CPU and RAM

OK since there isn't many uses for these parts I shoved them all into one step.

Processor-
OK to be honest you can't do alot with a processor but surprisingly you can make a hairbrush and a belt buckle.

1.Gold Plated Hair Brush
This ones pretty self explanatory, get a processor and glue it to a chunk of wood or metal; But heres the link anyway

See HERE (If your not bald)

2.A Geeky Belt Buckle
This ones quite cool but I wouldn't wear it as a everyday belt.

See HERE

_

RAM-
At first I didn't think there was much usage for RAM except put them all into one computer and put it to use as in the 1st step, but stranoster brought these to my attention.

1.RAM comb
To go with your processor hairbrush!

See HERE

2.Make it into a mood light
Surprisingly its possiable.

See HERE

Motherboard-
You may be asking me " What possibly can I do with a old motherboard, well the answer is not very much.

1.Make it into a bracelet
I don't really recommend this as the motherboard is full of poisious chemicals but Meh here is the link.

See HERE

2.Make little men/bugs!
This is very cool

See HERE

Step 7: Other Parts

Well not much left now except a few fans, the power switch and LEDs, a few PCI cards and some cables.

Fans
These are quite handy just to have lying around incase one in your proper computer packs up.

1. Hook it up to batteries
If you live in a area where it sometimes gets hot, these are very useful. Hook it up to a 9v battery or wire it up to USB power

See HERE
OR
See HERE (Battery Powered)

2.Make a hover craft
Isn't exactly the next age of personal transport but its a start. But don't use x3 9v battery's only use
one.

See HERE

3.Make a air filter
Not a bad idea...

See HERE

4.Make a better air filter
Better but a bit more complicated

See HERE

5.Rav them Out!

See HERE

6. Emergency-Fan-Power-Unit

See HERE

Power Switch And LED'S
They can't be really reused in a cool fashion but are useful as switches in other projects.

Step 8: Stuff That Has Been Built

Photo's of stuff that has been built from ideas on this instructables.

Step 9: The Case

Previously this page has no ideas except a picture of a posionious BBQ but thanks to MrJentis for sending me his instructable.

Make the case into a safe
Coolest use for a case yet, and somewhere to store valuables.

Add LED'S.
Self explainitory

Any other ideas, comment!