I agree in part with the first response. But the main point that I would say is NEVER execute one of the popup registry cleaners that often come up even if they look ok. Many will place things into your computer or into the registry that are hard to locate and hard to remove. Their solution is to sell you their program that will eliminate the millions of errors.
There are some free and widely used registry cleaners out there. I would not use any of these unless you have a problem that is driving you crazy.
Wise Registry Clean is a program that helps in some cases and really does no hard. In most cases, it will allow you to back up your registry prior to making changes. This is a good thing, if you find that your solution is worse that the problem, you can return to what you had.
Make and use registry backups. The registry is one place I do not like those I support to play around in. Microsoft has programs built into most operating systems that will allow someone to enter the registry and you can really cause problems. Unless you know what your doing, I would not recommend dabbling into the registry.
Since there are some good free cleaners, I never use any pay programs. I have fixed problems like stubborn popups and toolbars that may not show up in the program remove area on the control panel.
I have had to perform surgery on a registry that keeps having problems and no other way is out there without downloading an unknown program that might cause more problems than it helps.
So there is my two cents worth. You may find those who support this concept and other who do not. I don't always trust some advise from Microsoft since they really don't help the end user very well.