This problem could be a corrupted profile issue. But first try to run this command:
Start->run..then type the following -> “Outlook.exe /resetnavpane”
If that not works, then try to create a new profile and copy the old .pst file to the new one and look if it works.
If still have troubles after that, try to restore the system some point in the past you know it works fine.
And the last option could be a .pst corrupted file, search in microsoft site for a tool to fix Outlook pst files.
Hope it helps.