Optiplex gx280 doesn't recognize any serial or ps2 mouse

Please, can you help me?

 My Optiplex gx280 doesn't recognize at start up any ps2 or serial mouse and

it doesn't appear in Device  Manager...

Sometime it recognizes the serial mouse at start up and it works fine for all the time, but

it immediately freezes again when I change user.   In this istance the mouse appears in Device Manager,

I deactivate it and it's not possible to activate it again(it remains deactivated)

s.o.  xp sp3

The single ps2 port is good because it works fine with any ps2 keyboard.

Thanks