Based on the decsription, I'm assuming you are running NT3.51 in this control. The error sounds like it is having a problem loading the "Main" group. For lack of a better description, this is the folder that contains the links to run some of Windows standard applications (i.e. explorer).
Have you, or anyone else in your shop been going into the Windows interface? It's possible this group was inadvertently moved or deleted.
Do you have a backup of your hard drive? If you do, putting it in should get rid of the problem since it sounds exclusively Windows based.
If you don't have a backup hard drive, I'd consider making one now in case this is an indication your hard drive is starting to fail, just be sure to read all the warnings in the other threads on doing a backup before doing it yourself.
At a minimum, you should do a current backup of your machine dependent data immediately so if you have to replace the hard drive, you have a current copy of all your settings.