I have several problems with Half Life 2 on Vista. Textures are corrupted on some screens and I can hear sound lagging occasionally. Computer is far beyond recommended specifications for playing Half Life 2 so I guess it is not the problem. Other games are good. Can you tell what is wrong, if you have any information?
Be sure you have latest drivers for video card installed. Also visit their web site and download latest patches for Half Life 2 because this game had some issues with Vista if it is unpatched. Sound lagging is game engine issue and you can not do anything about that. It happens when game is loading large amount of data in video RAM.