Hi There, many, many, many things can cause this sort of thing. Just because you updated the antivirus doesn't necessarily mean you are not infected oh and did I forget to mention that some of these virus checkers can actually be the cause of the issue sometimes themselves. www.avast.com has a great one free to the home user, just as a test take the one you have completely off and try that one see if anything gets better (scan your drive with it). There are some viruses that no matter what you do, cannot be removed if running on the OS drive they got planted upon, i.e. you have to pull your hard drive out and scan it/devirus it with another machine than the one it was in...sometimes viruses can hide themselves too well in the OS to be cleaned out except when inert. You might also be loaded with spyware malware, get adaware free version or one of the other ones out there and clean all the malware off your hard drive.
This can also be a bad, or going bad memory stick. If you're comfortable taking the hood off your pc swap the sticks in the slots. Most likely if you've got a marginal chip the machine won't start with it in the lower slot (marked zero or one).
When was the last time you defragmented your hard drive? Swap files can get fragmented like any other file and it will cause issues, I really like diskkeeper it's a great program and can be set to automatically keep your c: drive defragged, not too expensive at around 40 bucks either. Another thing to checks is how much free space you've got on your drive, a nearly full drive can cause some of this too.
Something running in your system tray is sucking too many resources, or you just plain need to add memory or hard drive space.
This one is reaching but I've seen wierder things, when was the last time you checked the cmos backup battery on your motherboard? I've had people tell me their machines wouldn't start or worse just because this dumb battery was weak. I check mine on new motherboards, I've found more than one dead one right out of the box.
do you have a wireless router? Could your neighbors be wardriving off you and sucking off your bandwidth? On my belkin routers I have never had a problem with multiple computers hooked to it affecting other hookups speeds but with this netgear I've got right now I have definetly noticed that stuff happening so try unplugging any other machines hard wired into your router for the moment. I know, WEP is a pita but if your neighbors are sucking your connection that's your problem right there.