Does it start up and run?
Give as many details as possible, including what problem you are trying to solve specifically.
One way is to do a system restore, if the unit starts up and runs.
Another way is to use the XP install disk to do a system repair. The directions are posted in this forum, sixth post from the top.
If the unit does start, it is wise to back up all your personal files to a DVD, etc...before starting any repairs or restores.