SDK Not working

OK i know there is lots of threads on the internet about this, but using google with searchwords "Source SDK not launching starting" is getting too many fail threads, so I'm asking you any solution. It shows the window "Source SDK is preparing to launch..." and then Steam shows Im in-game and i gotta go to Ctrl+Alt+Del -> Processes to quit something i never saw. So how can i fix this problem. Reinstalling Steam is barely an option.