Okay, I've been fighting this problem for weeks and decided it was time for drastic actions. I backed up my data, formatted my hard drive, and re-installed a full version of Vista. Since I'm pretty sure this all started back in November, after putting on some "recommended" Vista updates, I only installed important, security-related updates and ignored everything else except the one for SP1. I also dropped the free versions of avast! and zone alarm that I had been usine in favor of zone alarm security suite (pay version).
It's been over two weeks with the computer up and running most of the time and the problem has not recurred and I have not had to reboot the system. It's not much of a root cause analysis but it seems to be a...