Can't even play, check.dll fail, help!

After downloading i click the shortcut to play, and i receive and error message as follows:

PatchClientDll ;; CheckDatFiles Failed. [Unable to Decrypt and de-serialize the data and download from the server].

I have no idea what to do. Please help. I can also post a screen shot if someone shows me how to link a personal picture.
My steam id is : SKULLTAKEN for anyone up for a quick reply.