your registry is your friend .... don't risk upsetting it.
I've been in the trade now for 20 odd years and read a mixture of popular and trade journals. In all that time I have NEVER EVER EVER (get the idea?) read a review of a registry cleaner. The reviews do mention general cleaners (CC Cleaner, window washer etc) but never include or mention the registry type.
the registry is dynamic .... some stuff is stored there for ever .... (hkclasses root, and local machine) some stuff changes according to the logged on user and some of it changes minute by minute (encryption seeds current docs).
so at any given time ... there is bound to be quite an amount that doesn't relate to the current state but does no harm
download regshot
http://regshot.blog.googlepages.com/regshot
run it 10 minutes apart to see what changes
microsoft say that there is no reason to do anything with the registry (unless it grows over 8Gb!!!) It's a flatfile database which are designed to be huge ... and very efficient
clean out your temp files and folders, defrag your disc
rebuild your machine every 12 months .... and that's the best you can do