Winxp ATI video 'freezing' issue

A friend of mine has run into a really strange problem that we can't
seem to track down a solution for.

Basically, whenever he opens a video file (any format, really) in any
player- the video will freeze up, UNLESS he moves his mouse, or has
another window on top of the video that is playing. Also, it seems to
be using 100% of his CPU (until he moves his mouse/has a window on top
of the video).

Now, only the actual video is freezing, the sound stays fine.

He currently has DX9.0b installed, and the latest drivers for his
video card.

And, just in case it matters, he just did a fresh install of WindowsXP
and has all of the current updates.

Any suggestions?

Thanks in advance,

Alex