Every time I open up Gmod, steam displays the common, "Preparing to launch" message, and right before closing, it shows a hollow window very briefly and Garrys mod on the taskbar. The game never starts. I'm having close to the same problem with my source game,:TF2.
I know for a fact this laptop has had an unstable background, and I recently had to install XP and then Vista to get it up and running again. It's not mine personally, I would never take such terrible care of my comp back home. I've tried so many things to no avail, including launch commands.