I personally use AVG for anti-virus it is all that I really need. but anyways, yeah sounds like you may have some spy-ware.. that or you just installed WAY too much on the poor thing.(or something is just hogging memory).

And for IE.. ditch it... I don't care what else you use, but any computer I fix I get rid of IE.. and you know what it lives much longer then with IE.. so I don't like IE. I personally use Opera over Firefox.. didn't like firefox much when I tried.. go out and try new browsers and see what you like.