Flash videos play really choppy

I just can't figure it out. I have an old PC (P2 350 with 256 ram -- Win2K Pro) and my flash videos run so choppy (even in a smaller screen). My P4 at work can play them no problem. What baffles me is that I'm able to play avi's (movies) in fullscreen with no slowdown whatsoever on my old PC. It's obvious that flash videos require a faster CPU in order to run smooth. Why is this? I was so surprised to see my avi files play in fullscreen with no hiccups.

Can someone explain why flash vids are CPU hogs?? What's the difference between avi's and flash vids?

Thanks.