a really good registry cleaner is regvac from superwin software. it does much the same as regclean, but is more thorough. it also has other goodies you may like. i also use jv16 power tools. the most complete reg cleaner and sometimes a little scary to use is nbg clean registry(russian).
regclean is not meant to be used on winxp. i would be hesitant to use it. i also have norton utilities. windoctor is not a registry cleaner. it looks for specific errors in the registry. regvac can be used on xp and does a better job than regclean. the registry cleaner in system mechanic is not a cpmplete registry cleaner, as you will notice by how fast it runs. it looks for specific things. try nbg clean registry. you dont have to delete registry entries, but you will see just how much junk is in your registry.
I'll wager that Powertools is at least as good as and probably better than the other three and it won't cost you a dime. The guy who wrote Powertools has been doing it a long time. It used to be called RegCleaner but is much improved and does more things.
I can see why MS would be recommending System Mechanic. At $60 a pop, they must be getting a piece of the action.
I did the google search for erundt. Came up empty. Links would help those interested.