My Computer Folder has 'access denied' error message

A few PCs at work running Windows XP are all having the same issue:

When attempting to access the hard drives (a 160 gig drive partitioned into an 80 gig C: and 80 gig D through the My Computer icon, I get a pop-up "Access Denied" error message when I click on one of the drives. This is only on the hard drives though as I'm able to access the DVD burner via My Computer. I am able to access the hard drives through Windows Explorer, however.

This problem is under the administrator's login, so I wouldn't think it's a user rights management issue.

The company that provides my workplace with Windows image discs/licenses seems absolutely clueless about what could be causing this, and since the PCs in question are non-networked PCs used in the field instead of office, they can't net me any help either. Any insight to this problem would be greatly appreciated.