Ok, dit is wat je moet doen, ga naar:
http://www.steampowered.com/index.php?area=news
en ga ongeveer naar het midden van de pagina, waar dit stukje teskt staat:
A small number of Half-Life 2 customers have been experiencing problems while playing the game, and we have traced these issues to corrupt Steam cache files. We've released a Steam update to help resolve these issues. If you have been experiencing problems such as crashing during gameplay or startup of Half-Life 2, please restart Steam and then click on one of the links below. Steam will then automatically verify your cache files. The verification process will take roughly five minutes to complete.
Klik dat op validate HL²
Steam opnieuw opstarten, en voila, het werkt, net zelf gedaan.