EN7950 video freeze problem

I'm having a strange and frustrating problem and was wondering if anyone here can offer any suggestions.

Whenever I play a video file my computer freezes after a while. It doesn't matter which format (I have tested .xvid, .mpg, .avi and .wmv) or which player (VLC fares slightly better than WMP which tends to crash immediately - VLC will usually go a few minutes). The computer requires a reset to get out of it, though occasionally it will reset itself.

There is nothing in the error logs (control panel / administrative tools) reported from either program, or indeed anything else out of the ordinary. I don't think it's a heat issue as the computer will happily play games for hours with no apparent issues and is well ventilated and well cooled. The notoriously inaccurate Asus PC Probe shows the CPU temperature as being 31C/87F and the motherboard temperature as being 38C/100F, which sounds about right.

In addition (though I'm not sure if it's related), when I start the computer up just before Windows explorer finally starts (i.e after the black XP loading screen but before the mouse pointer appears) I get a weird blocky screen which appears then goes away after a second or two, and things go on as normal.
With apologies for poor camerawork:

The computer is an E6600 running on an Asus P5W DH Deluxe with bios revision 1101 (the latest on Asus' site). It has 2Gb of Corsair RAM and an Asus EN7950 GX2 card with the latest Forceware. It's running XP MCE with DirectX 9.0c and the latest drivers for chipset, audio and so on, as well as all the XP updates that Microsoft can throw at it. I don't think there's anything wrong with the video card, as it seems quite capable of comparatively difficult things such as gaming and 3D mark stuff.

Anyone else had this problem? Any suggestions?

Disable video bios cache, diable fastwrites, disable assign irq for vga change refresh rate to adapter default, disable hault on errors.
Did you use the Avic changer to change the xvid to use mpg4 codes or anyother format.