Instructables
Firstly, Download a free programme called Ccleaner, what it does it deletes all your cookies, history on your computer, So make sure any websites you want you add to favourites.

In this Picture i have deleted over  200mb, once i got over 2.5gb deleted. thats alot of memory for my laptop.
 
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Step 1: Empty the recycle bin

Make sure you empty your recycle bin as it may have big files in there you dont want.

Step 2: End any programme

end any programme you dont need as it takes up time from where you are using it. unless you are downloading anything from a torrent site like vuze if you are downloading something ;)

Step 4: Upgrade your ramm

Upgrading your ramm will make you compuer/laptop alot faster. getting the right one for your operating system is the thing so. go to a website called www.crucial.com to find out
ewcrew6 months ago

in total I deleted about one and a quarter Giga bytes. my computer was full of crap I didn't need.

ewcrew6 months ago

I deleted over 1GB just by emptying my recycling bin

psmurf562 years ago
If you don't want to worry about this just run Linux! ;)
TDILLT4 years ago
If running windows, there are tonnes of tmp files just hidden,
also if you don't delete them, they are called "system" files and just
clog up your drive.

Defragmenting your drive works as well after deleting temp files.

I could go on, but to truly make your computer faster, you have to care for it.
If you want to find a more expanded version, I've been working on my guide for a few years, and it's at it's final release. This is much more in depth, take a look: http://www.instructables.com/id/Several-Easy-Steps-to-Boost-Your-Computers-Speed/
jdege4 years ago
Actually, at this point the most significant improvement to performance you can make is to upgrade to SSD drives.
This program has been covered here many times...

Don't forget, after uninstalling any unused programs to run CCleaner again. Also run the registry cleaner in CCleaner. I am a computer repair/network engineer. I use this on almost every computer I get in front of. Cleaning the registry is the best thing you can do to speed up your computer. Most bang for the buck type of thing.

If you have deeper issues, the best paid app I have found is System Mechanic Pro. You can catch it on sale occasionally for $39.95 . If you dig on the iolo.com website, you can find a 30 day demo. 

Remember if you like it to purchase it. It will also help keep some of the new spyware/malware off your computer.