With alot of trial and error, I've found out that Steam in general but especially Counterstrike Source has alot of problems when Internet Download Manager is installed on the same system.
It causes the game to freeze upon loading a map in lets say 95% of the time. I've tried to narrow it down and stumbled upon this download manager that I was using.
I would suggest not to use it or to uninstall it. I'm trying to narrow down what part of the program is causing it and if older versions are experiencing the same problems.
This was tested on my own system. Specs can be found in my profile, on a Windows 7 64 bit Build 7100 with all updates done.
More info can be obtained on my blog (dutch) Kenneth's World!
or follow me on twitter @kdegussem