I think this is beyond System Restore, and it may even be hardware related, but in order to rule out Windows, you'll need a valid cd. Can you borrow one of the same flavor from a friend? You can use it if you know your original product key
Belarc will tell you if you don't remember.http://www.belarc.co...e_download.html
If this is a pirated copy of XP, this is not an option for you, and we need to know so we don't spin our wheels.
If you have recently added new hardware, please post what it was, and have a look in your event logs, too. We need more clues in order to be able to help.