From a newsgroup post:
Problem: In Outlook 2003, I have an additional "personal folders" item, which is empty and cannot be deleted. When I click on it, I get the message: "the set of folders cannot be opened" and the when I click help I get the following:
"This error usually appears if the OST or PST file you are using is:
- Protected with file permissions
- On a share on a server and the network is down
- Corrupt. To correct this problem store the OST or PST file on the local computer. You may also want to run the scanost.exe and scanpst.exe tools to verify file integrity”.
Solution: First, make sure that the .PST file is not set as your default mail delivery location. If it's not and you can't close it by right-clicking its root folder and selecting Close or by going into File -> Data File Management and removing it that way, then go to Control Panel -> Mail -> Show Profiles and create a new profile from scratch containing just your good .PST.