I use alt-tab in civ4 and it works fine. Unlike a lot of games it's not disabled, and it doesn't seem to cause any instability issues either.
If your computer freezes from alt-tabbing its probably because your pc doesn't have enough ram or cpu power to get away with it, as alt-tabbing out of games is a bit of a dodgy process, and it puts a huge stress on both of those parts for a short while. Only really high spec pcs can alt-tab quickly out of a demanding program like civ.