This is a tricky one since there are so many registry cleaners/optimizers on the market now that trying to find the one that really does what it says it can do is a headache
I think the best way is trial and error but from my past experiences the smaller the app the better!! I use CCleaner alot which is a free download and have used Vista Optimizer which seems to do the job! sorry i cant be more specific but like i said trial and a lot of errors!! lol.
Hope this helps??