Do you have the mouseware installed? You can switch between USB mouse and PS/2 mouse in that, and also configure the buttons. (I don't know how you're mant to set it without a working mouse though... like a monitor manual in PDF format)
I gave up with the USB Logitech keyboard/mouse as they worked intermittently, not using PS/2 for both of mine.
Had problems initially on XP with the supplied drivers, but a download of Mouseware & drivers from the Logitech website fixed them.