Do you have the original XP installation disk? Or your computer restore disk?
Have you tried to run system restore, setting the drive back to a time before the problem started? Restore does not erase your personal files and is reversible.
click all programs
click system tools
click system restore
check the box that says restore my computer to an earlier time, click next
select a bold date on the calender, a date before the problem started, click next
follow through with the restore directions
restart the computer after it finishes
if the system restore does not work, select an even earlier bold date on the calender and repeat the procedure
try this with at least four earlier dates before giving up and trying something else