Registry cleaners work by searching for potentially obsolete entries. They don't necessarily, or usually, constitute errors, per se, and cleaning them up doesn't offer any real performance increase on modern systems. Now and then they'll help by removing old entries that are preventing the associated program from being reinstalled or causing erroneous behavior, but they're also prone to removing valid entries that can cause programs, and even Windows, not to work. Thus, it's a mixed bag, and I'd never pay for a registry cleaner. Instead, I run RegSeeker (freeware) every once in a while, which is comparable to many of the paid cleaners. None of them catch everything, but it's actually better to err on the side of caution than maximize the removal of keys and end up causing a problem instead of solving one.
Ok so my computer has been running quite slow recently so I checked the forums for a Registry Cleaner to see if it would help any. I downloaded this free trial of Registry Fix to soon find out I had over 1000 errors on my computer. I was quite surprised. So I bought the key which was $40.00... and cleaned it all up... I guess. Things I guess seem to be working fairly good again, I don't know.... Anyways, I thought I would download another program just because and see what it said. I downloaded at random Netcom 3, which does the same thing. Turns out I still have over 600 errors?????? Even after my other program I paid 40 bucks for said I was error free... So basically I hate life now.