Those are "UserAssist" items. They increase in number in the registry as windows records what programs you use, so it can construct a frequently used program list on the start menu.
It looks like PowerTools may be calling many or even all of them "does not exist" errors.
Normally, those registry items are encoded using a "ROT13" encryption. It seems the Windows 7 beta has "temporarily" changed to use a different method. That means jouni has to make a temporary change to accommodate Windows 7 beta testers. I suspect that won't be a real high priority.
What you can do in the meantime is add explorer\userassist to your Registry Cleaner's "Ignore words" tab.
The quote below is from this page: http://windowsir.blogspot.com/2009/01/catching-up.html
Didier found out recently that theVigenere encryption used in the UserAssist keys in Window 7 are only to be used in the beta, and the "encryption" method for the final version is going to go back to good ol' ROT-13!
Here's another reference: http://blog.didierstevens.com/2009/01/18/quickpost-windows-7-beta-rot13-replaced-with-vigenere-great-joke/