Registry cleaners are extremely powerful applications and their potential for harming your OS far outweighs any small potential for improving your computer's performance.
There are a number of them available and some are more safe than others. Keep in mind that no two registry cleaners work entirely the same way. Each vendor uses different criteria as to what constitutes a "bad" entry. One cleaner may find entries on your system that will not cause a problem when removed, another may not find the same entries, and still another may want to remove entries required for a program to work. Without research into what the registry entry selected for deletion is, a registry cleaner can end up being an automated method to cause problems with the registry.
For routine use by those not familiar with the registry, the benefits to your computer are negligible while the potential risks are great.
Further reading:
XP Fixes Myth #1: Registry Cleaners
