A lot of Registry cleaners can be dangerous but one which is safe because it only skims over the files, removing only the unnecessary entries is CCleaner from http://www.piriform.com. However, the Registry is a small folder of files in the greater scheme of things and hardly affects speed at all.
Use CCleaner first to remocve all the clutter in the system before scanning the Registry and when offered the chance to back it up first, accept that offer.
When your systemis runing slowly, bring up the Task Manager by pressing Control, SHift and Escape at the same time. Look under the Processes tab, click twice on the top of the CPU column to bring the highest users to the top and see what's taking up the resources.
I would recommend against even CCleaner. CCleaner is intended to be a precision tool for people who fully understand the pros and cons of each option, not a panacea tool for virtually everything that ails a computer.
With a few very specific exceptions, the registry needs no cleaning or maintenance of any kind from the user. It is far more likely to cause problems using ANY kind of registry cleaner, than resolve them.