I sometimes open the forum and can't believe the questions we're getting.
Game programming is a popular entry point into the world of computation for young people, for obvious reasons. I think we should keep that in mind, that not everyone posting here has any well of experience to draw from.
Vista ships with all frameworks up to 3.0. W7 ships with 3.5. Windows XP is still the dominant OS out there, with about 2/3 of machines, but the various .NET Frameworks went on the "critical updates" list a few months ago so it's on most XP machines.
So, generally, no, it won't take up any space than it has to for your own compiled code. Just don't go too far with unnecessary libraries...