Originally Posted by Adraco

Right, how large is your .pst file?
Assuming you run Windows it should be somewhere here: c:\documents and settings\USERNAME\local settings\application data and then either the Microsoft or Outlook folder.

Reboot computer, after it boots back up, shut down all crap which runs in the background and then try again.

Try to increase your page file, thus creating more virtual memory or just buy more RAM.