I'm not sure if this is why other people were having warhammer crashes, it's a really strange problem in fact. To start with, here's the hardware in my windows box:

Gigabyte GA-MA770-DS3 motherboard (ick)
AMD Athlon64 X2 6000+ @ 3.1GHz
Geil 2GiB (2x 1GiB) dual-channel DDR2-800, CAS-4 RAM
ATI Radeon HD 4850

Firstly, I remembered that this memory can run at 1.9v-2.3v, optimum being 2.1v, and the board was defaulting to 1.8v. After failing to find a way of changing the voltage in the BIOS, I gave up and used the crappy overclocking tool to change the voltage to 2.1v when windows boots. Still crashes, someone in ##Hardware suggests underclocking the RAM to DDR2-667.... SUCCESS!

It's been several days now and I've not had a single crash, I can't figure out why I had to underclock the RAM, as 3dmark 06 ran fine, and memtest86 did a 12h test cycle without crashing or finding any errors, warhammer just somehow makes the hardware fuckup badly when this particular setup is running at DDR2-800. If anyone else is still having the reboot/poweroff issue, try this out and let me know how it goes.