I use Ace Utilities regularly, and highly recommend it as the safest utility I've found for cleaning the registry - as well as it's other functions.
As installed - with default settings - it uses a time-tested 'exceptions list', to prevent removal of critical registry entries that might cripple your system. It also has a menu that allows an 'expert' mode, if you wish to operate without the exceptions list.
The only instance of registry-cleaning problem I've encountered with AU is the deletion of registry entries for 'radrun', which are necessary for operation of my Omega graphics card drivers. I added those items to the exceptions list - problem solved.
You will be prompted to set a restore point before using the program, and any registry-cleaning operations you wish to restore can easily be restored via the AU menu.
If you've purchased Ace Utilities, you are entitled to a free download of their RC registry-compression program also.
Hope this helps!
Edited by gud4u, 02 January 2005 - 10:05 PM.