OK. I have what seems to me to be a rather bizarre problem. I'm running Vista Ultimate x64 and use AnyDVD HD to rip Blu-rays to my HDD as .m2ts files. I have installed "K-Lite Codec Pack Full 4.1.7" and "K-Lite Codec Pack 64-bit 1.4.0" so that i can play them in WMP11. So if i just open one of them, King Arthur for instance, it will open in a windowed mode and play perfectly. But then when I make it fullscreen, it will cause my PC to hang immediately, which forces a restart. The same thing happens if I open the files in Nero 8, MPC, and PowerDVD 8. All hardware drivers are updated.
I also have a Vista Ultimate x86 install on the same PC and I have yet to experience this problem so I think that my hardware is sound.