Have you tried reinstalling Firefox because if the Profile folder is in tacked it will load the Bookmarks automatically.
If they are anywhere they are in your Profile folder, have a look in:
C:\Documents and Settings\[User Name]\Application Data\Mozilla\Firefox\Profiles
Have a rummage around in the folders in there, there's usually about three folders with random numbers, you are looking for Bookmarks.html
Having said that don't surprised if you can't find them because by reinstalling XP it might have overwritten the folder, but it's worth a try.