Try removing the folders through the control panel:

1. Close Outlook.
2. Click Start > Control Panel.
3. Click Mail (you may have to change the View By to Small icons or Large icons)
4. Click Data Files.
5. Select the .pst file that you no longer need, and click Remove.

This will remove the .pst file from your Outlook profile, but wont delete it from you computer (just in case you still need it).

Hope this helps.

-SW