I agree that CC Cleaner is excellent but I would advise you to be very cautious about using registry cleaners. If you have a known problem then they can be very useful but there is always the danger that they could do more harm than good.
Have you had a look at optimizing Windows 7 - have a look at this article and have a go at a bit of the "housekeeping" it suggests.
It is particularly important that you keep drivers up to date and have the latest versions of installed programmes.
The registry holds every setting, all configurations, and basically tells the system what to do. If the registry is messed up then the system can be made inoperable.
There are times when you have to resort to a registry clean and a back up should always be made - where you should be cautious is using them on a regular basis for no apparent reason. Have a look at this article which goes into the pitfalls in great dept.
CC Cleaner does perform a registry clean but it has other functions too - you can uninstall unwanted programmes from there and generally clear temporary internet files, cookies etc.
I am attaching an article about optimizing XP - these are all recognised tweaks and you can decide which, if any, you want to do.
I fully agree with Terry Brown, difarn and onthelimit1. CCleaner is a very good tool in general, and it's registry cleaning function has never caused me a problem. On the other hand other more agressive registry cleaners I have tried have caused severe problems. I strongly urge you to heed the advice the others have given.
If you like to keep things "tight" , it may be worth using Revo (free) uninstaller to uninstall progrannes as it seems to clear out more "debris" than the Windows one.