I've recently started plowing through HL2 and the episodic content (currently starting out on Ep 2), and have noticed a weird problem.
It seems that after an undetermined amount of time, my computer will crash. My computer is a notebook with an Nvidia dedicated graphics chip...its good enough to run HL2 and Fallout 3 (but as I'm away from it now, I don't have the specs handy).
I can combat this crashing issue by periodically alt+tabbing to my desktop and letting my computer relax for a bit.
On another note, my fans are completely clear when I play...airflow should not be a problem IMO.
On another another note, I have absolutely zero problems playing TF2 or CSS....TF2 will lag for me slightly if there is too much going on at once (i.e. 32 player mayhem), but no crashing.