If you don't want to go with the pre-made solutions, my rudimentary understanding is as follows
Under Linux, you can just use a regular H Bridge like the one detailed on this site and then write bits to the serial port to turn the control lines high or low, just like if you were running it offa mcu. You can find numerous tutorials on the net detailing writing and reading from the serial port. http://www.societyofrobots.com/robotforum/index.php?topic=5729.0
I wrote an example in the above post about writing to the parallel port so if you've got one of those, it might be a good alternative to a serial port.
Under windows 2000 and later things get a bit more complicated because of the port security features of the kernel, and you're better off using a commercial controller that comes with software because otherwise your going to have to write a kernel level driver.
One more thing. You should be careful with your serial port, chucking some octocouplers between the h bridge and the serial port will reduce the chance of frying your serial bus.