computer vs. me: i won!
well, it took giving in and doing a complete wipe of my machine, but i've finally fixed the damn laptop. naturally, during the systematic reinstallation of all the applications i use, the problem came back and i was actually able to find the root cause. AND IT COULD HAPPEN TO YOU!!!
dramatic overtones aside, this was a pain in the ass and the fix is easy. sure, it turns out i didn't have to wipe the machine, but from my pain i pass on my knowledge. the short and dirty is this: on vista and win7 (at the least) zone alarm (freebie version) and norton anti-virus 2010 do not play nicely together. the symptoms are random encoding errors in any browser and all downloaded files erroring out as corrupted (or "not a win32 program"). neither norton nor malwarebytes ever found a virus and the errors did not surface running in safe mode.
anyway, long story short, the problem reared its head as soon as i reinstalled zone alarm. a quick (and more finely tuned than my prior) google search found the same symptoms noted in the zone alarm support forums. i gave the latest beta for zone alarm a try, but it doesn't fix the issue, so it had to go.