I had this problem a couple times while playing counterstrike and half life. I read that having both on board sound and an additional sound card installed at the same time can lead to this problem. I disabled my on board sound and haven't had another lockup/freeze up yet...

Heres the page. (http://steampowered.custhelp.com/cgi-bin/steampowered.cfg/php/enduser/popup_adp.php?p_sid=*jD2Uorh&p_lva=280&p_li=&p_faqid=150&p_created=1093320749)