Watchdog Timer

I am trying to design a watchdog timer for a win XP machine, this consists of a relay board with a countdown timer which will give out a short circuit to the reset pins on the mother board after about 5 minutes thus resetting the machine if windows should hang etc.

The relay board will stop it's countdown and reset to the 5 minute value if it gets a pulse, say 5 to 12 volts. My question is where can I get this pulse? My first thought would be from a com port pin but I'm finding it hard to get any information on what protocols win XP assigns to the com port and whether this protocol my change if windows hangs, fails to load or loads in safe mode.

I was thinking of the DTR pin but I think that is only in operation when a modem is plugged in.

Any help from you hardware wizards would be greatly appreciated.