Howdy oharaandrewlp,
We suggest AVG becuse they do have a free version and it takes up the least amount of resources on your PC compared to Norton etc. I use the paid version and it picks everything off before it hits the gate.
Hijackthis is just a toold for when you get malware and such for users that know how to use that program. It gives a read out of allot of what is running on your pc so we can see what malware or viruses you do have on your pc. I would not suggest that if you do not know how to use
HJT do not use it, becuase in the wrong hands it could kill your windows install and you would have to reinstall it.
As far as computers slowing down over the months that is a natural thing. Your computer gets bloated wth crud and temp files etc.. Get fragmented.
Have you used a program like Cleanup?
Download it in my sig and run it. It helps speed things along as well as a good defrag program
|MG| Diskeeper Lite 7.0 Build 418 is a good defrag program better than windows one..and free