If it still boots to Windows and you can burn a disk or two (CD or DVD), copy everything you can to it. My Documents, Program Files, Favourites, etc. If you have a few email addresses you want to keep, print out the address book to hard copy or file and save that too. There's also the option of using the Files and Settings Wizard in XP, this will save all the things you want to keep to a file and can be restored later. (The only drawback is you might take the problem with you if its in the stuff you saved.)
Then do the format and re-install Windows.
Leave plenty of time, this is not a fast process
Bill