I seem to have the same problem..... I'm well into the campaign and at some point in the game the end turn button stops working... I may have found a way to bypass this. I will post the info when I conform this.
I'm running windows vista 64 bit version and I have the same issue.
I had a different problem ( the game would suddenly crash with no errors), setting it to window mode fixed it. I now have the out of memory crash ( I also run it on 4GB RAM 64bit vista) :(
The problem I'm encountering is the game suddenly crashing ( no error windows just crashes) is this the same problem?