You can go Control Panel->Programs->View Installed Updates to find out exactly what update was installed. (You can sort by the date installed.) There was only one update released recently, KB925902, which was designed to patch the ANI cursor exploit. If that's not it, I would consider a driver update for your graphics card to be at fault. Try installing the latest drivers from ATI. If that doesn't resolve it, try a System Restore to a time just prior to the update's installation.
Aside from that, launch Problem Reports and Solutions from the Control Panel and see if any other crash occurred at about the same time Windows shutdown unexpectedly. That tends to be a useful way of finding the root of the problem without trudging through the Event Logs.
Ok, I have absolutly no problem with playing games on Home Premium, ASUS mobo with ATI Radeon X1300 and AMD Athlon 64 last night (everything played just fine, so I'm nearly if not absolultly, completly sure that is not the cause here), but I just learned that it downloaded and installed some update for Vista, not sure what (it's a few files, so not sure which one is the incriminating one), but ever since then, whenever I tried to run a game, it'll play for about a minute to 5, then my entire system will crash and reboot. It did this before with the update (don't know if it's the same update, but it downloaded and updated something, and when installed, did the same thing). This is not with one specific game, because I tried a few games, and it is doing this to all of them.
I repeat, though, just in case one may think it's a card issue or the mobo or whatever (no matter what one may say, they'll be people who will still try to point it to there), THIS NEVER HAPPENED BEFORE THE UPDATE WAS INSTALLED!! I just don't know the exact Microsoft update file that is the culprit, or if there has been anyone else with the problem after this update was installed, or what to do to fix it.