error message:

"Cannot display the folder. The file
C:\Users\Shelly\AppData\Local\Microsoft\Outlook\Ou tlook.pst is in use and
cannot be accessed. Close any application that is using this file, and then
try again. You might need to restart your computer."

I've tried several times with no luck. What do I do now?!?

using:
- Windows Vista
- Outlook 2007