Look at http://www.quancom.de they offer fairly affordable and reliable USB, PCI and PCI-e watchdogs. Their cheapest USB watchdog costs just €50 or so. I got an older PCI card made by them off eBay for just €25.

I'm not triggering the watchdog in the kernel or a background demon process. Instead I modfied the Phoenix miner to trigger the card every time a result is accepted by a pool (the modification is less than 20 lines of code). Together with a timeout of a bit more than one minute, this rather brutal approach works splendidly - no matter what fails (except power obviously), as soon as no results go to the pool for a while, the box does a hard reboot. Of course you had better use a journaling file system with such a setup. My stripped down Windows 7 setup with NTFS will be mining again just two minutes and a few seconds after the original failure.

As for a serial port - if your mainboard doesn't have a built in serial port or a serial pin header (some still do even today) you can get an USB to serial dongle. Check that it uses a chip that's supported on Linux - most should work these days. Then just point your software at /dev/ttyUSB0 or whatever it's called.