1) back up ALL your data.
Data files from Word, Excel etc.
whatever else you can think of.
2) Make sure you Dl the latest Mobo drivers etc. for your board from the Manufacturers website, and also for your Modem.
imho you don't necessarily need all the drivers for all your cards etc. as XP has a HUGE database of drivers, AND if you're connected to the internet, it will go and search for more from windows update. Motherboard and Modem are essential.
Once you've backed up and Dled drivers. Turn off the PC, go away for a few hours. Come back and check. You'll prolly remember something !
3) whack in the disk and choose clean install - pref NTFS.
XP will do the rest.
You'll have to re-install your programs, but XP should take care of the drivers.