I run WIN2K and have the EXACT same problem. It is not APPS running in the background. Or ANY of the other things people have mentioned. (And for GODSAKE, DO NOT shut EVERYTHING down as one person suggested. There are some important system threads running that you want to leave alone. Make sure you know what you're shutting down before you do it.)

I've tried it all. The problem is particularly apparent after importing ANY Poser animation. (see my previous post) What I discovered was that closing the timeline (yes, the timeline) makes a huge difference. (I know, a lot of good that does when you're trying to animate something.) Also sometimes maximizing the main window, moving something around you will notice it is smoother, than restore the main window and it works better.

Best of luck!

-Tim