Crazy Mouse

My mouse will move up and down but not side to side.
I have checked for viruses, installed the latest video drivers, installed a known good mouse, reinstalled Win98SE…still no side to side movement. It is a PS2 mouse so I tried a serial mouse…same thing.
There are no conflicts in the device manager. I have not installed any new software. The power supply failed and I replaced that. Could the failed power supply have damaged the motherboard or chip? Why would this symptom happen with both PS2 and serial mice?
AMD K6-II 500, 64MB ram, Diamond SIII 540 AGP video, Win98SE.
Thanks for any help.