RTT = Real Time Tactics. This is not to be confused with RTS.
Anyway, the game page is
here and a direct download is
here.
It should work on Windows 2000, XP, and Vista, and is probably also fine on ME, 98, and 7.
So what's it about? Well, RTT is a pretty good description; it's like an RTS, but the focus is on the micromanagement of a small squad of units, right down to controlling when they fire. There are no buildings, no resources, none of that stuff; just very involving, tactical combat.
Screenshot (actual game has a much larger view):
(I'm not required to use the template, right?)