This is what I found out in my own personal tests.
If I play any files encoded in the MOV file format that are stored LOCALLY on any of my 3 internal SATA HD's, the file play for a few seconds with sound, but without video, then eventually the buffer overrun error occurs.
If I stream the EXACT SAME file via the internet, or my local home network, or play them from my external USB HD, USB Thumb drive, or even burn them to CD/DVD and play them from that, I do not encounter the buffer overrun problem, and the files play just fine, even the files that are in high definition. I'm not an expert, but in my case, this may point to poor Nvidia SATA drivers, although I could be wrong.
Just to recap. If I play any MOV files from the internal SATA HD's attached to my motherboard directly, I get a problem. If I stream them, or play them from another storage medium, I do not encounter a problem.
Not sure if this really helps anyone else, but I though it might be useful for others to try out.
QX6700 ASUS Striker Extreme Windows Vista