Alright, I've run a spyware scan (clean), uninstalled and reinstalled IE7, ensured that Shockwave and Flash Player are up-to-date, and deleted TIFs (although I did so via Disk Cleanup just a couple days ago as well). Now all there is to do is wait, I suppose; the problem is intermittent, and could take 2 or 3 hours of pure web browsing to show up if it's still around. Since I have more important things in life than concentrated web browsing, I'll just make sure to post here again if it recurs.
Oh, and I'm running Norton Internet Security 2005, sure, it's old and a performance hog, but, long story short, I've got 330 days of (accidentally) free subscription time left, and this computer is basically a stopgap until I get back to the States and get some money to build a new PC, so I'm not going to bother upgrading Norton, even if it is the cause of this only marginally disruptive problem, which I doubt, since it didn't happen on my old XP install with the same version of Norton.
...and that was a really long sentence. Good day.