Media player keeps minimizing

homepc

I am using Windows Media Player 9 and lately when I am watching any movie at
full screen mode after about 10 - 20 min it goes back to maximized display
with the menu bar displayed above. The same happens when I am running Media
Player Classic. I am using Windows XP home and have Zone Alarm Security
Suite running in the back ground.

Usually the movie seems to slow down and speed up just before it happens.
After I select full screen mode again it's usually good for the rest of the
playback. I am using FFDSHOW for a video codec. I have tired turning the
libavcodec on and off in the FFDSHOW configuration but it makes no
difference.

This started up around the same time I updated Windows XP to SP3, but I
cannot confirm if this is what caused it.

Anyone have similar problem?