Just got a report from Valve about the annoying Vista hl2.exe crash when you close any Source game on a Windows Vista machine. It seems Valve has found a solution and are trying to push it out hopefully with the next TF2 update.

Basically when running Windows Vista (mostly x64) and you close a Steam (hl2) game like Counter-Strike: Source or Team Fortress 2, Windows Vista comes up with a pop saying that hl2.exe crashed. This is very annoying for a lot of people and hopefully soon the fix will be out.