You say you have the latest BIOS? Have you updated it from the original or something?
Anyway, Advanced Configuration and Power Interface (ACPI)
Perhaps you could look in the BIOS and see if there is a setting you need to alter.
Or you could load BIOS defaults or take out motherboard battery for 10 minutes to reset it to default.
also, has it always had Vista on or have you upgraded from XP?
and what make and model of pc is it?