It's actually surprisingly easy too and works for Outlook 2003, 2007 and 2010.
Outlook 2010 Method
- Make sure Outlook is closed.
- Open Windows Explorer and browse to %APPDATA%\Microsoft\Outlook
- Rename Outlook.nk2 to outlook nk2.bak (just in case you need it ever again)
- Thats it. There is no step 4 :-)