Well with just Firefox or IE open I usually don't have an issue. I also usually have quite a few tabs open as long as I don't have the other programs running. But in the last few months I've started using pretty heavy programs (Dreamweaver, Photoshop, Excel with large files, 150meg+ pdf files...) and that'* when the problems came up.
I do a CD backup and also store select important files on a secure remote server. That way if there'* ever a fire I'm still in good shape.
I run Adaware, Spybot *&D, and BPS in rotations weekly, not just installed. I also run HijackThis roughly montly just to check a few things I could catch. I also run a virus checker montly. I have a host of other tweaking programs (registry cleaners and what-not) I run every so often and do a smart placement defrag with Perfect disk every day.
I try to keep the computer running as smoothly as possible, that'* another reason this problem is so frustrating. I keep thinking about upgrading again, but for what I do this comp is generally fast enough if not for this issue.
I'm running just a tad more than I usually do (no problems yet!) but here'* my task manager screenshot incase anything sticks out to someone: