Blank screen when starting GMod


I have a problem with GMod freezing on a blank screen when I start it up. After clicking icon and before anything is displayed.

I have GMOD 10 running on Windows 7 Ultimate x64 with 4gb RAM (1600mhz DDR3) on Intel I7 920 with Gigabyte GTX260 video card, and gigabyte GA-EX58-UD3R mobo.

I have also tried running it in Windows XP Sp2 compatability mode, as suggested by the windows compatability assistant, but that did not work either.

Really dont want to downgrade to x86 because all other computers in house are running Win 7 x64, and we can’t share devices arcoss 32/64 bit systems.

Can anyone suggest a solution?