System hard lock-ups

Specs first

Win 7 64 pro
i7 3930K
MSI x79a-gd65 (8d)
12 gigs of Patriot RAM
Gigabyte hd6990
Corsair 1200 PSU

Zonealarm firewall pro
MSE anitvir

I'm experiencing strange lock-ups. The screen will freeze and no input from the mouse or keyboard will get through, no sound either. There's no visual glitching, no looping or screeching sound, the hdd led is off, the fans work - the screen just freezes what it was showing at the moment and stays that way until I do a hard reboot. So far this happened several times when using Photoshop, several times while watching flash videos and a bunch of time in games, namely Dota2, Anno 2070 and once in Blacklight retribution.

So far I did the following in a bit of a backward order

- checked another PSU that I know is good (no change)
- checked another GPU (a 5990) that I know is good (no change)
- checked RAM, including sticks from another set that I know is good (no change)
- updated a bunch of drivers for chipset, Realtek sound and network - found out I can't use the latest network drivers from MSI's site, they'll install correctly but after a reboot I get a driver installation failed message - could be that the network card is borked?
- did a clean reinstall of Win7 (no change)
- changed Catalyst from 12.6 to last stable version 12.1 - this got Dota2 from freezing upon loading to not freezing at all. Anno 2070 still locks about 10-15 minutes into the game. Blacklight does not lock at all after the driver change and it locked up only once before on 12.6

The temps are all in order. Once I managed to get a lockup while the task manager was up, neither CPU nor memory capped out, both at 12% and about 2.9 gig of load respectively.

I have literally no idea what is going on here.