My friends computer has become infected with spyware or a virus. He is running Windows XP on a Compaq. Theres a little dialog box popping up constantly that says theres a virus on the computer. When you click on the box it redirects you to a spyware remover page. Obviously its an attempt to sell this product. How can I remove this and stop all these pop ups? I was thinking about manually cleaning the registry. How would I go about that? And what about those programs in the Add/Remove programs in control panel that just wont go away? I need to get this junk off his computer. What do I do? Without spending money that is.

I want to speed my comp up get rid of all files which I can not find and I need someone help who know what they are talking about. Like I do not want to have to buy something for it when I know there is a way to do it without spending money so please may someone show me how and steps of what i have to do ?
How to de-frag your hardrive