a) Whats the specification of your computer?
e.g. processor, motherboard model, amount of memory, graphics card (if separate to the motherboard).
b) Did you use the options in the WinXP setup program to reformat? or did you do it a different way?
c) If you DID use the WinXP setup to format the drive did you also complete remove and recreate the partition before reformatting?
d) If you have access to another hard drive, try installing the Harddrive into the machine and install XP Pro on that. That will tell you if the problem is hard drive related or not.