Hi Ray! Thanx for your answer. My guess was that there wouldn't be a problem as RegVac run well in Vista. The registry structure/construction seems to be similar to Vista and XP. But for safety, I first created a System restore point, then installed RegVac. Before doing a cleanup I also made a registry backup via RegVac, then I ran a Clean Registry. RegVac found loads of redundants to clean up (as expected).
After rebooting the computer and trying to run RegVac again I for some reason had to use the Program compability function (via right-click), choosed "as Vista" and then it ran perfecfly .
By the way; Windows 7 seems great, I've thrown out Vista from two computers, installed good old XP on one and now tried the beta version on Windows 7 on the other, I really like it. Some bugs of course, but it's a beta and still better than Vista from my point of view.