Why Is My Computer Running Slow

My slow computer

My computer is reading an error saying that Windows security Center has detected spyware/adware infection and it recommends that I use special antispyware tools to prevent data loss. I don’t know what to do. I have Norton so I thought things like this would be prevented and on top of everything else it is now sooooo very slow! What should I do? I don’t really trust letting anyone fix on my computer since this all happened a couple of months after I had it fixed for another infection that caused nasty pop ups!
I am able to get onto the internet and it’s working fine. It’s just my desktop programs that are slow. (word, any icon that I click, everything on the desktop) M y AVG did pick up a malware I also have something called Clean Up but they are all running slowly and if I leave to wait on it, when I come back it’s gone. The clean up program is only running in demo mode so I’m trying to get that going. I am trying some of the things you guys listed below. Thanks so much!