I use Advanced Windows Care and TweakNow Registry Cleaner (both free).
Recently I've been looking for something better. I 've bought about 3 or 4 and returned them. I tried about 10 trials. Some of them find a whole lot more errors than mine, but after I clean I don't notice any difference in performance. Actually some seemed to slow me down.
With the vast knowledge in cyberspace someone should be able to steer me in the right direction. Please:)