My computer must have a virus

I'm not very savvy when it comes to the computer, but I think I must have a virus or something, because my browsers keep on opening and closing by themselves, and even as I was looking to post this message, the screen changed to another section. I have ran NIS multiple times and nothing but tracking cookies is found. I ran the program listed at the end of the scan which says to run if I think there's still a problem, then I also ran a program found here in the Norton forums called the Norton Power Eraser.

So far nothing has been found, but I can't use the computer without having to maximize my windows all the time. Does anyone know what to do in this instance? I tried looking in the startup programs, but I'm not sure what to do. I found a couple of things which I thought shouldn't be there, but nothing solved. Help please? Thank you!!!