this might fix it. go to game properties then go to local files and press verify integrity of game cache. that might work.
Well, that didn't work either :/.
But thanks for trying to help
EDIT :
I managed to get it back running by modifying the resolution keys in the registry and setting them to a low value
(1024 * 768)
But still, I don't understand why it wouldn't launch, as it didn't crash before stopping to work...
RE-EDIT :
It has stopped working again, I have no clue why
RE-RE-EDIT :
Launching it in windows XP SP3 seems to do the trick... For now...
Sorry for editing this much, but I think it prevents from "flooding" the topic.