NOTE  These simplified instructions were taken from Microsoft KnowledgeBase Article 287497, available here:

http://support.microsoft.com/kb/287497

Run the Inbox Repair Tool

1. Click Start and point to Find, or Search, depending upon your operating system. 
2. In the Search for files or folders named box, type scanost.exe. 
3. Double click the Scanost.exe file to open the program. 
4. Choose your Outlook profile & hit OK
5. Sign in with your e-mail address & Outlook password
6. Make sure Scan all folders is selected
7. Make sure Repair errors is selected
8. Click Begin Scan.

The scan may take a while to run and repair any errors.  Performing this operation, especially on large mailboxes, greatly reduces the chance for errors.