What is the registry? The registry acts as the traffic cop of a Windows operating system. When your computer needs to accomplish task it looks to the registry for instructions on how to do it. The registry can hold hundreds of thousands of commands, and every time you do one simple thing, even double clicking an icon with your mouse to open a program, the operating system looks at the registry to know what to do with the icon once it's double clicked.
Seem mindboggling? Think of this as well. You buy a brand new computer, get it home and get it up and running. You're gleefully surprised at how fast it is. Oddly, six or seven months later, you're complaining about how slow your computer is and what a piece of crap. Well, the reason it's so slow is because the registry has become expanded since the day you bought it. Also, when you uninstall a program, sometimes all the registry entries are not clear, and leaves what is called orphaned registries. They don't lead to anything, and they make the registry bloated. Sometimes these entries can become corrupted for any number of reasons. These all contribute to the computer slowing down, and not just a little bit either. Combine all of this with maybe having a virus or worm on your computer and you can really get slowed down.
The next time you log on to your computer and think it's slow, think about downloading a registry cleaner and gain some of your performance back!