You only want to mess around with your registry if there is something wrong with it...and even then, under guided supervision! Please try to avoid registry cleaners as well...you're really not going to gain much (if anything at all), and you take an extremely high risk of borking your machine by using them.
A slow computer is not a reason to try and clean your registry. Your best advice is to look elsewhere on your computer to fight poor performance, such as following some points in the page Alexstrasza linked to above.
...And if your computer is still performing slow after you've done some steps in that link, then feel free to post a new topic in the Am I Infected? forum for further assistance.
If you have any other questions about what could be the cause, don't hesitate to ask there as well! That's really what the subforum is designed for.
Best of luck,