I have had similar problems that turned out to be a USB driver issue with Vista 64.
Get the latest drivers from your mainboard manufacturers website and post more details about your system.
What version of Windows are you using?
Does it happen with any other USB devices?
Do you know what power supply is in the computer?
Do you have any other USB devices plugged in at the time this occurs?
I did some further investigation myself and it turned out in the meantime, that the USB port is broken and is giving a short-circuit signal, which makes the PC shut down.
Am positive that this is the problem - will get a new internal port and then we'll see.
Thanks for your help.
Software (bugs) should not be able to turn the system off instantly; so this would indeed be more consistent with a hardware situation. I myself have experienced this, with inserting a USB pendrive which was wet at its connectors. Upon insertion i got a nasty surprise.