Assuming this desktop is worthless as a computer . . .
CPU heatsink and fan MIGHT handle a small H Bridge. Switching power supply could make a 12v/5v bench supply, case might make local scrapper a nice gift. Great rare earth magnets in HDD, if really old then small useful steppers in 5" floppy drive. Hang HDD platters in fruit trees to scare birds . . .
If it works, then it could be the brains, but you will need some programming skills or a friend who has them. Parallel and serial ports CAN do useful things, just think of a printer as a robot.
But now you are talking about some serious batteries to haul around, or have the desktop control the robot via 802.11 wireless link . . .