hi Mark. I'm not certain about this, but first I'd run a disk check.
.... right-click C:\ drive in My Computer
.... go to Tools tab
.... select scan disk & check both boxes to fix errors & replace bad boot
sectors.
In Windows XP the screen should automatically ask if you'd like to schedule
a scan upon restart. Click "ok."
If that doesn't do it, I'd reinstall Outlook AFTER backing up your pst file.
Hope that's of help!