[Suiseiseki]

For some reason when I try to play Stalker, it restarts my computer. It started doing it at the beginning of the second level. It also sometimes completely freezes up, and forces me to hard reset. I just bought it off Steam no more than a week ago, so it's all up to date. The official website has no tech support, or really any type of support for the game at all. My computer's only ever done this while playing the Painkiller BOOH expansion, where it just reboots itself while I'm playing. I'm well within the minimum system requirements, I run Windows XP, have a decent video card, a gig of RAM, and enough hard drive space. Any suggestions?