I run Vista and yes, it is a pretty big problem. However HL2.exe error is caused by multiple things.
I'm sure you've tried this but verify the integrity of game cache. Also defragment cache files.
Also have you tried running steam as an administrator? If so then try this:
Go to the command prompt (hit run and type in cmd.exe)
and type this in:
net localgroup Administrators /add Local Service
then you can type this in:
net localgroup Administrators
and if LOCAL NT AUTHORITY/SERVICE is listed, then it worked. Now go reboot.
If nothing else I think you are able to run steam in compatibility mode. Change it to XP, service pack as high as it can go and boom.
Sometimes it just crashes because it's mentally challenged, it happens to everyone. I'll go to my gmod folder to see I have like 50 crash reports sitting in it that takes up like 30 megs. (I think I got up to like 200 megs worth of crash reports once).