Why do we need to clean up the registry? What is registry? What is it use for? Where does it come from?

I found a trojan and some spyware on the registry of my PC. Would a reinstall restore the registry to when I first bought it? The thing is, I've done that, and the trojan and spyware still shows up in the registry keys when I do a scan.

Hi i have had to system restore and system recovery quite a lot lately so ive uninstalled and reinstalled lots of stuff

As a result some of my software and games are behaving abnormally and i am getting unusual error meseages

How can i clean up my registry for free

Any other advice you can give me

Thanks

I have registry cleaner and I seen a free trail for registry defender..cleaned says I have no problems and defender says I have 154……

Ok registry cleaners are not free, the scan is but the help isn't. Are the issues the scan reveals really valid and can I fix these problems myself? My Pc runs slow and hangs up often, it stops responding. What really causes this? Is Norton 360 a good virus and spyware product? I've defragged, optimized and fully run virus/ spyware scans. What more is there to do?