Load up Process Explorer, make sure you have I/O Delta Total column showing, and sort, this will show all the processes consuming disk IO in real time. See what processes are eating up both CPU and disk.
Antivirus software is worse than a virus IMO... I'd uninstall it completely and just do a online web scan once in a while. I will absolutely have no software on my computer that behaves as you've described. Get to know your process list inside out, including dlls, download only trusted files (eg: WoW addons are that only WoW addon scripts in a zip file, no .exes for example), and uninstall your antivirus.
Keep an eye on Process Explorer from time to time and monitor running processes, and use a online virus scan software to double check your vigilance. Particularly do both immediately if you know you're intentionally doing something risky like opening exes or anything from P2P sources, etc.
Your hard disk drive servo will thank you and reward you with 10 fold the longevity and performance.