Is the installation finalizing when you launch it for the first time, you might see a .net distribution package run (
iirc) as it talks to Steam after a complete reinstall ?
If this process fails it will never seem launch whatever you do.
But....
Some have had success reinstalling the distribution packages in the Steam folder, and you might want to update your .net 4 installation.
http://www.microsoft.com/en-us/downl....aspx?id=17851
.