Every so often I get a complete system lock with this message in event viewer:
ACPI: ACPI BIOS is attempting to write to an illegal PCI Operation Region (0xc), Please contact your system vendor for technical assistance.
I have seen it with several windows open or nothing running at all.
I am running Window 7 Beta 64bit, an HP XW8400, with a quad core xeon and 8GB of ram, with the lastest bios. I have turned off all power management options and it is still locking up.