Most important here I would suggest you is check if all the floppies (if booting from floppies) or CDs and drivers are in working condition. Coz this happened to me, wanted to repair XP and my 6th bootable floopy did not work. Since I had a back up of the floppy, it solved the problem. So,
backup and verify the data before doing anything. If just upgrading, everything should go on smoothly..
