Uhm well, that error can be caused by many different things, so is hard to know what is the exact problem of the crash, but your HL2 is working fine, so i think naybe is game related and not Steam/OS related but im not sure.
I assume that you are using Win Vista because if im not wrong that message is from that OS, what make more difficult to assist you because i donґt know about vista-steam problems so i gonna concentrate in the game itself.
First try if you haveґt already the basics methods:
"Try to run the game in administrator mode / Do a verify cache files with steam / make sure you have the minimum requeriments to run the game".
After that if continue crashing try this:
1- Run the game throug the .exe instead of Steam, it maybe will show you the same error or another different but just try.
2- Put your Steam.dll where the .exe is and try again.
3- If you have addons added in the Gmod directory delete them.
4- Copy and reemplace the hl2.exe with the hl2.exe of your HL2 game.
5- Delete the Gmod folder and let Steam create it again.
6- If you have a VM with Win-XP or another computer with Win-XP installed try to run the game with it.
7- Try another crack or launcher to run the game.
Tell us if something change.