BSOD (or complete freeze) mostly when playing L4D2

Hi, I've been getting random BSOD during last 4 months. There's not much in common among them, lately it happens when I play Left4Dead2, but since it's the only game I play it does not say much. Previous BSODs were sometimes related to (un)mounting SD card in card reader or external hard drive, but I can't say there is any direct connection. Today it happened twice. First I got L4D freeze...