L4D2 had/has a windows permissions problem for some folks. The fix for the CSC file in this thread worked for me as well as others here.
CTD: (VISTA 64-BIT)
* ** START > MY COMPUTER > WINDOWS > RIGHT CLICK 'CSC' > PROPERTIES > SECURITY | Make the ADMINISTRATOR own the folder.
Using Win7 Ultimate btw + there are other fixes in that thread.