It's not as simple at it used to be. Today's laptops are 'meant' for Vista, so trying to install XP often isn't easy. In the case of my 5620, the hard drive wasn't even detected until I researched and changed Bios settings (known as the AHCI issue). I'm SO much happier after ditching Vista, though- when I first got it, a 1GHz Pentium III with XP could have run circles around it!
Step 1: Preparation
First, a Full Backup of your system. Trust me, if you ever have to send your Acer in for servicing, it better have Vista on it or your warranty could be voided. Plus, someday when you sell it, uninformed people think it's a plus point ;)
Secondly, why bother downloading random drivers you hope are the right ones? Acer makes it easy with its Driver and Application backup CD creator. A little known fact is that these drivers appear to be both the complete Vista AND XP set. Once you're done, you'll have burned three DVD's, and be ready for some XP goodness! If you've already wiped it, or just want the latest available, grab them from [ftp://ftp.support.acer-euro.com/notebook/ here]. I did both, just in case, throwing the newest onto a 256MB USB drive. Thirdly, look in Device Manager to see what AHCI Disk Controller you have! Write this down. On the Best Buy Acer Extensa 5620-6830, it's the 'Intel 82801HEM/HBM SATA AHCI.' Other models may be slightly different. This info is important later, and it can be difficult to find out without some annoying trial and error.
Step 2: Make the Bios compatible
Step 3: XP Installation
Step 4: Driver setup
Step 5: Fixing AHCI
Now we manually install the driver into XP: Go to Device Manager, under IDE ATA/ATAPI controllers you should see something like: ICH8M SATA Controller. Right-Click on that and select Update Driver. Choose No to connecting to Windows Update to search, then hit Next. Select Install from a list or specific location (Advanced), hit Next, then select "Don't search. I will choose the driver to install." Hit Next again, then select Have Disk. Browse to your Drivers folder (C:\Driver), highlight the iastor.inf file, and hit Open. Select OK, then choose your AHCI driver (what we wrote down earlier, remember?). You'll likely have to uncheck the 'show compatible hardware' option to see the AHCI options. Again, on the Extensa 5620-6830, it's the Intel 82801HEM/HBM SATA AHCI Storage Controller- YMMV (Your Motherboard May Vary). Click next, ignore the warning that installing the device driver is not recommended, click Yes, Finish, then Yes to restart your computer.
Step 6: Finishing & Troubleshooting / Tips
Troubleshooting: If you still get a blue screen upon booting up, you may not have selected the correct AHCI driver. Reverting the Bios setting to IDE should get you back into XP to try again. If not, use F8 to go into Safe Mode and reinstall the controller driver there.
Tips: I ran into an issue where my favorite OpenGL screensavers won't run over 1fps on the 5620. After finding out that this is why (Intel disables OGL hardware acceleration on screensavers), I then found a workaround: rename screensavers to *.sCr instead of all lowercase. You may have to reboot, but then they should all work smooth again. This affects all X3100 users, or anyone with Intel GPU's.
Step 7: You're done!
The screenshot below is my Acer's desktop now. The next Instructable I post will explain how I got it to look like Vista, and some other tweaks you can do to speed up this or any other computer. Hope you found this helpful :)