It came with its own little adapter and yet, the performance on the serial port is very inferior. I have not checked however, if Windows (XP) loads the same driver when I connect it through the serial port as when I connect it through the PS/2. I'll check tonight when I get back home.
The ideal situation for me here, would be if the trackball worked on the serial as well as it does on the PS/2. This way, I would not need to purchase anything else.
Thanks for the answers so far.