Hi Murray
Try again, with a fresh download and installation of the Microsoft Installer Cleanup Utility directly from Microsoft ---
http://support.microsoft.com/kb/290301
It shouldn't prompt for System Administrator unless you are using Win2000/XP/Vista. If you continue to have trouble with that, you could try CCleaner as an alternative, it's the only other tool I trust for Registry work (we don't recommend the casual use of Registry Cleaners - but your situation calls for such a tool).
Best of luck
. . . Gary