GARRYSMOD launch error

im having a trouble whit my gmod 10 when i try to start it says;
failed to load the launcher DLL:
%1 is not a valid win32 program

i have a css so it should work xD …soo if anyone can help me plzz do

Have you played before?

I seem to remember this happening with 64 bit systems or something.
By the way, your GMod is legit right?