My computer is messed up. I don’t have the system recovery CD, so does anyone know how to clean up a computer and basically restart it as when I just bought it?

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