Jason one of the main Valve developers has mentioned many times the "hl2.exe has stopped working" error is nothing but a cosmetic bug caused by WHEA (Windows Error Architecture), the glitch is caused when it unloads the internal encrypted file system used within Steam without Vista knowing it got unloaded and Windows detects the wrong exit code for something thats "not there" so it generates the error...
Its
nothing more than a cosmetic bug you can bypass by adding hl2.exe to the "Problem Reports and Solutions" exception list from within your control panel, Valve will eventually remove the glitch when Microsoft gets around to changing its WHEA code and Valve cant do it themselves without a complete rewrite of the entire Steam codebase and encryption systems.
There is no real problem with Steam or Vista and this would have been fixed ages ago but Microsoft blames Valve and Valve blames Microsoft...
I do know another solution for making the error disappear by adding your user account to a special group already built-into Vista but its not advisable.
Steven