I agree with Steve, in terms of not paying for the software CCleaner is one of the best plus it does just a litte but more than the registry. You can also defrag the registry with a problem called RegDefrag from Auslogistics. They also make a nice disk defrag problem also which I find better than the built in Windows utility.
ASO is the best of everything, even includes a registry defrag. However, if all you want is a registry cleaner then the ARO single version by itself is pretty awesome as well.