Hi Jaki,
Try a repair installation of XP -
Microsoft: Windows XP Pro FAQ - Tek-Tips - print the page out and follow it through.
Or take a look at
How to Repair Install:
and
Perform a Repair Installation
and
Repair XP too
Pick the one you like the look of to follow. Basically, you are just refreshing the Windows files - any updates will need to be installed afterwards but all your programs and files should remain intact.
Do you think this might be time to rethink your backup strategy
?