Your problem might be because you are running on a low-end machine. I played most of my games on a sub-spec rig (emulating DX 9 instead of running it natively) and saw a *major* change when I finally tried the game on an OK video card running on a fast CPU with decent RAM.
A couple of things you might try other than buying Vista too soon:
* Be sure you're running the latest update for the game and your video drivers
* Don't leave the map zoomed in to detail level when you process a turn; all those polygons slow things down (this can help, but isn't a cure)
If you've already tried that, I fear hardware might be your only fix (other than seriously practicing patience). Before the new rig, I always got to a point like this in my gigantic map games. I also suspect that player ships moving through AI sensor coverage is a factor, which could be why the zoom out only helps some and why better CPU & RAM could help.
p.s. Making your subject line a bit more specific might have drawn attention more effectively, e.g. "Small problem with long turn processing."