There is no demo of GOW for the PC. It's a fun game, but you have to enjoy a very "console-like" experience. (No quicksaves, 3rd person only, boss battles, etc.)
If you want to see what it's like, there are plenty of gameplay videos on youtube, and maybe you could convince your local GameStop (etc.) to let you see it on the XBox. (It's really not much different on the PC, save for the controls, unless you use a gamepad.)
