About 1 in 3 startups, Vista will boot, run for a few minutes, then just stop. It doesn't BSOD, it just freezes up (mouse cursor, sound and everything). I have to hard reset the computer using the power button.
This is not a hardware problem, as I dual boot Ubuntu and that doesn't have similar problems despite being used far more often.
It's also unlikely to be a driver problem, as Windows has ran fine for a year up until this started about 3 weeks ago.
A (possibly related) oddity: After Linux installs some kernel updates, it prompts me to change the grub boot file. I leave it do a 3 way merge. The next boot, Vista takes significantly longer to boot (4-5 minutes as opposed to 1 minute).
EDIT: The System log shows nothing except the "unexpected shutdown" from me holding the power button to switch it off.