WMV 720 HD videos choppy?

I have an Athlon XP 2500+ Oc'd to 3200 speeds, a 6600GT 128mb AGP 8x, 1gig of PC-3200 ram

And yet 720p HD videos are choppy. What can I do?

Upgrade processor. It might be fast but it doesn't have multimedia instruction sets used in modern processors which might result in extremely bad performance on certain tasks.

As for serious advice, do you have a recent video card driver? Are you watching the video in overlay mode which is accelerated by graphics card or not? I have same graphics card as you and a 2ghz processor, the video from that site in 720p runs perfect in media player classic using ffdshow with postprocessing enabled. You can try to disable postprocessing aswell.