I play in win 7 x64 as well, and it doesn't crash on me when alt-tabbing, but it acts like it does. Are you talking about it crashing when you go back to the greyed-out screen and it says "PoN is not responding"? If so, you have to click "wait for a response". It will take a little while as it switches between AI turns and the supply phase, but it will go back to the game after one of these phase switches.
If this isn't what you're talking about, my apologies.