I've have had many PCs come in that were acting strange or were very slow. Many had virus infections and spyware and the users would tell me "but I have virus programs".
In many cases I've seen multiple programs running at the same time. A big no-no!
I have also noted that McAfee and Norton Security software greatly slowing down a PC and see the CPU being chewed up in Task Manager. Now add 2 or 3 of these programs and run them on one PC and you get a very slow responding PC.
OK - first thing I do is UNinstall ALL of them, reboot the PC ( maybe have to do that a couple times ) then download a decent antispyware program ( I like Superantispyware - Portable ) run that - and then Uninstall that too.
Finally - we like AVAST - Free - used to like AVG but they have gotten "bloated" lately.
Anyway - AVAST has served us well and we have had very little complaints.