I've been doing some investigating; no answers, yet, but here's what I've done.
I installed Nod32 on my GX280/Vista SP2/IE8/KB972260 and rebooted. I was able to log on an moments later my system was locked.
I booted into Safe Mode w/ Networking and installed Process Monitor, and configured a boot logging session.
Rebooted normally, and though my system was sluggish, it did respond for a while. Eventually, it stopped responding. I reset the machine, booted to safe mode, and examined the procmon log. I didn't see anything obvious among the 1.8 M entries...
I disabled the "ESET Service" service, and rebooted. System came up and has been responsive. There was a hung ESET splash-screen, which I identified as the ESET egui user mode application (the little eset icon in the system tray). I killed that process. And the system has been fine for the past 15-20 minutes.
Regarding FREE Antivirus:
I use AVG on my home systems; but beware that the installer "offers" (will install by default) the myAsk toolbar, which is some cruftware that no one needs. Be careful when looking at the installation dialogs of these apps:
Sun Java installer "offers" a rotating set of apps, including a trial version of McAfee.
Adobe installers "offer" the google toolbar.
Regarding contacting ESET: We're working on it
Regarding cached creds: XP doesn't support cached creds in safe mode; Vista does. Yay Vista.