Instead of PC Cillin, I use Ad-Aware for spyware. AVG Free is good for antivirus. Also... if you have someone who's willing to evaluate the scans it does... there's a program out called hi-jack this. It scans your pc's running processes and searches for files that the name is there but the file itself is missing, and all sorts of other stuff. If the problems with the computer being slow persist, find Hi-Jack This, run a system scan, and send the results to me in a PM here. I'll go through and tell you what it's coming up with and how to fix what needs to be fixed.
DO NOT, however, try and mess with the results yourself. Hi-Jack has a tendency to override natural security measures windows has in place to keep it operating and if you select to fix/delete the wrong thing, you can crash windows and any other number of things can be messed up.