This error was either:
- RGSC does not start, shows nothing.
- Windows shows "RGSC has encountered a problem and needs to close" before starting RGSC (this is not the same error as RGSC "The application has encountered a fatal error...", that error happens after RGSC starts succesfully)
Ok, to the solution:
1 - Uninstall all the .NET framework runtimes, from last version to first (for example, I did 3.5 first, then 3.0, then 2.0. I don't know if this really matters, but to be sure, do it this way) You may not have all the versions installed. If you have .NET 1.0 and/or 1.1, it's not necessary to uninstall it. People using Vista cannot uninstall .NET 2.0 and 3.0, but it's ok.
1a - Vista Only: Go to the services console ( start - run - and type "services.msc"), look for "Microsoft .NET Framework NGEN x64 and/or x86". right click on them, stop them, then on properties set them to "disabled".
2 - Install .NET framework 3.5 SP1 ( link )
2a - XP Only: Right after installing it, go to the services console ( start - run - and type "services.msc"), look for the ".NET Runtime Optimization Service", right click on it, stop it, then on properties set it to "disabled".
3 - Launch RGSC.
Note: Make sure you aren't running RGSC in any type of compatibility mode, and that you're running RGSCLauncher.exe instead of the other executables.
This error also makes the new Games For Windows Client (the one that you can launch from windows without being in a game) crash on startup, so if you get that problem too this should solve it.