PS/2 Trackball won't let computer power down

So I've searched all over about this problem and I can't seem to find any help. I have a PS/2 trackball plugged into my computer and it works great, but when I shut the computer off through Windows XP, Windows will shut down and the computer screen will turn itself off but the computer will stay on. The computer stays on indefinitely unless I unplug the trackball, at which point it shuts off instantly (which is what I wanted it to do in the first place). So it seems like the trackball is keeping the computer on. I was reading a few other forums and some people were saying that the PS/2 drivers weren't being terminated properly or something like that. Anyone have any ideas? I went through all the ACPI and APM settings and they all check out, and everything works fine with the mouse unplugged. Replacing the trackball with a different PS/2 mouse solves the problem, but I need to use the trackball. Thanks in advance for any help!