Have seen this 2 different times with two different problems.
1. Make sure your video card drivers are updated. My friend had this problem and once he updated his drivers problem was solved. Even tho ATI had no mention of CS:S in the Catalyst 5.7 release it seems to run much better on his and mine with the update (not to mention his doesnt crash out or shutdown anymore)
2. This one is probably just me but still mad because i dont have the cash anymore to upgrade. My video card heats up and shuts down... Stupid video card.
Number 1 is probably your fix... Now that i think about it, if you have a Creative sound card make sure all your drivers and updated for that. Have read about problems from their cards with CS:S. Hope that helps. BTW, welcome to
PR.
P.S. You may not have an ATI card but if you have an Nvidia make sure you updated drivers as well, just not sure if they have a new driver release.