1. Close Outlook.
2. Run a search for the file Outbox.pst (or, if you're using Outlook Express, Outbox.dbx)
3. Right click on it and select delete.
4. Restart your PC. (I'm not sure that's essential but I always do it when fixing these type of problems in Outlook or Outlook Express)
5. Open Outlook. It will search for all of the essential files associated with it, including Outbox.pst. When it can't find any of the files, Outlook simply creates a new one, so you'll have a brand new (and completely empty) Outbox.
Chris