Counter Strike Source crashes on launch after Migrating from Vista to 7

problem: After migrating from Vista to 7, CS:S no longer launches

You have Migrated from Vista to 7. I personally chose to clean install 7, as my games and other personal files are located on different hard drives.

When your system is ready for gaming, you load Steam, and when you try to launch CS:S, you see that hl2.exe launches but quickly dies. The game does not actually load 3D mode, so its not a crash to desktop, it just bails out on launching the hl2.exe while still on the desktop.

solution: force resolution and DirectX level

In the Counter Strike Source game properties, from the Steam games list, set the launch options as follows:

-dxlevel 90 -w 1024 -h 768

This should force CS:S to sort things out, with whatever is causing it to crash on launch. You can set your preferred settings in-game, quit the game, remove the launch options and then test to ensure everything is back to normal.

It is possible that my issue was specifically with CS:S remembering the wrong display adapter reference, perhaps it was not finding the reference to the previously known adaptor and getting its pants in a twist?

Its worth noting that the issue was CS:S specific in my case. Other hl2.exe based games work working fine.

Prior to trying this solution, I tried a bunch of other generic troubleshooting steps, including trying the -32bit launch option (given that I'm running x64 Windows 7). I also tried moving my existing cstrike cfg, to see if defaults would help. None of my attempts worked.

What MIGHT of worked, which I only learned about post finding the solution, was the -autoconfig launch option.

Feel free to read more about -dxlevel and -w -h launch options at the Valve developer community.
For specific info on dxlevel (mat_dxlevel) check out the Value developer community page for that cvar.