Try repairing your .pst file
Two suggestions (do in this order):
1. Run the scanpst.exe utility and see if your Outlook storage file has become corrupted (not uncommon). This utility will detect errors in the file, offer to make a backup (do it) and attempt to repair the file. You can find this file here:
C:\Program Files\Common Files\System\MSMAPI\1033
assuming you're using XP and Outlook 2003. The path may be different on an older OS and/or version of Outlook in which case try doing a search for files with the extension ".exe" and pick from the results. I suggest pinning this utility to your Start Menu for easy access in the future..
2. Run Detect and Repair from the Help menu in Outlook. You will most likely need your original Outlook CD to do this.
Hope that helps.