I have the same exact issue, running Vista Business 32bit, and instead of ZA, im running McAfee VirusScan Enterprise 8.5.0i.

Even when i leave for lunch, i return to my PC and see the same message you do about the unexpected shutdown.

i've given up lookin for a solution, but man its a pain in the ass when i leave things open for a quick lunch to return an have them all closed down.

the only help i've found so far seems to be sound/video card issues. vista did just ask to update during this write up so i'll see if these updates solve the problem.