How to send data from external device to OS with rs232 and fake it as a mouse
You could pretend to be a Microsoft Serial Mouse:
http://paulbourke.net/dataformats/serialmouse/

Edit: if that doesn't work, and the computer has a PS/2 mouse port, you can use that: http://www.computer-engineering.org/ps2protocol/

Otherwise you're going to have to get a microcontroller with USB support and learn that, which will be time-consuming.