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.