To be honest, obsolete registry entries don't really slow your computer down unless you're approaching six or seven figures. Each one takes up an extremely small amount of space, and thus is insignificant when compared to the accumulation of junk files (Temps, cookies, etc). However, every little bit does count, and cleaning your registry (with a good, respectible utility) rarely does anything but help your PC. Plus, in some circumstances having old registry entries can caus problems reinstalling/upgrading software, so it's usually a good idea to run a quick scan ever now and then.
Personally, I use RegCleaner, which is a free registry cleaner from TweakNow that has done well for me. Each month is finds a couple hundred obsolete entries and has never recommended the deletion of anything that it shouldn't have. Given that, I don't feel the need to have anything else. (I wouldn't bother purchasing the Professional version, for despite its advanced features, it barely found anything on my PCs that the free version didn't.)
I've used Registry Mechanic before, but had a mixed experience. It is by far one of the most thorough registry cleaners available, finding nearly 800 entries that RegCleaner, and several other utilities, missed. However, in going through the lists I found that around half of the entries listed under "deep scan" were not obsolete, with most of them belonging to Microsoft Encarta and Microsoft Money. Since the trial version prevents you from deselecting entries set to be deleted, and locks the options menu, I chose to stop there and not risk having those programs corrupted. (If these abilities were not disabled, I would have simply deselected the entries that I wanted to keep, then continues forward.) As it was, I just didn't think the risk was worth it and decided to stick with a program I know won't do more harm than good.
Hope this helps,
John
P.S. With any registry cleaner, it's always wise to back up your registry prior to launching the program. If the program doesn't prompt you to back your registry, go start->run->regedit and then go file->export and under "export range" at the bottom, slect all.
After reading many posts and articles about registry cleaners, I decided to download Registry Mechanic and run a scan. It claims I have over 1000 problems that need to be fixed. My computer seems to be running fine. It is a little slower but I attributed that to all of the antivirus and antispyware programs required today.
My question is, can you have that many problems with your registry and your computer runs OK? Should I clean out what it finds? I haven't used a registry cleaner since the one Microsoft offered free for Windows 95 and 98. I've backed up my registry in case I decide to do this but, I have some doubts whether it is a good idea or not. Any thoughts?
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