AVG
Norton
McAfee
CA
These are all resource hog programs.
They can and will slow your system down even when you are not doing a scan.
The free programs we here at GTG recommend are just as good and in most cases better then the above.
They are very easy on the resources and thus will not add to slowdown you may be experiencing.
How is your system running now after all the steps and hopefully uninstalling AVG?