Your graphics card may not be powerful enough or your processor isn't fast enough you may want to check this.

I also had this problem with another game but same principle it would run but when something that took a lot of power the computer would crash the game to desktop or just freeze the image and become unresponsive. 

you should also look at the software compatability list for windows 7 online because galciv II was only meant to be run on Win XP or Win Vista.

Hope this helps