I have Office 2007 installed with the cache file in C:
Left or right clicking on C:\MSOCache I get:
Quote
Directory Opus
An error occurred reading folder:
Access is denied.(5)
Retry Parent Abort
Unquote
This is the only directory in C: that gives this problem.
When I run DO9 as Administrator then no problem
Any ideas? 
It's because that folder's permissions don't allow non-administrators to even read the list of files in the folder.
Opus is slightly different to Explorer in this case. If you right-click a folder that you have no access to at all then you'll be shown an error message. In Explorer I think you either don't get a menu at all or you do get a cut-down menu.