CCleaner is great, and not just for deleting unreferenced registry entries.

(Sorry if this is a little OT.)

I don't like registry cleaners, in general. I've never had registry cleaning make a noticeable impact on system performance, but they can occasionally break things. Risks outweigh benefits, IMO.

Usually, when trying to speed a system up, I have better luck clearing out temp files, defragging, and using CCleaner/SpyBot/HijackThis!* to get rid of stuff that's wasting HD space, memory, or CPU cycles.

Also, if you get a fresh PC, PC Decrapifier is kind of nice. It checks stuff on your PC against a list of software most savvy people don't want on their computer and gives you the option to uninstall it. (Note: After it does that, it goes to a list of installed programs, like you can find in the control panel, and lets you uninstall things that aren't on that list, so don't go nuts and uninstall EVERYTHING just because it appeared in the program.)

*Note: This is an old version from before Trend Micro bought HT! and bloated it.

P.S. When installing free software, always pay attention to the installation options, so that you don't get stuck with crapware like toolbars from Yahoo!/Ask/etc.

My coworker fixes computers as his second job. He uses malwarebytes and Zdelete religiously. Zdelete is a miracle....not sure if you can get it for free but you can google it and try.