I think it must have something to do with the autorun.inf virus. I have made an automated tool for fixing the problem.
It deletes all instance of the autorun.inf files and disables scripting as well
It takes care of the condition when Task Manager and regedit is disabled!
all you have to do is run a small file that i made.
check it out: http://andback.wordpress.com/2008/04/19/restore-access-drives-under-my-computer/
To permanently fix the problem http://andback.wordpress.com/2008/03/20/removing-the-flashdrive-autoruninf-virus-v13/
I hope it helps!
Anurag
April 2008