PS2/USB problem

My father has Windows XP home edition on his PC and here's the situation.
He bought a new mouse because he thought he needed one since his optical mouse wasn't working.
I got over there to install it, and his keyboard wasn't working either. Both are PS2.
So, being there was no way to input anything to the computer I was stumped momentarily and left.
I phoned him back again from a friend's house and told him to try something out.
It occured to me to have him take the USB-to-PS2 adapter off of the new mouse cable and plug the USB end directly into the USB port where his printer was plugged in. It worked, and his keyboard which was still plugged into the other PS2 port magically came back on and is working.
I went back over there and was able to enable his PS2 port again for the mouse connection but when I tried plugging his printer back into the USB port it had come out of I ran into a snag.
The problem is this. When I plug his printer back into either available USB port it cuts the connection to the mouse, and keyboard. Both the mouse and keyboard lights don't come on, and neither one works in any way.
As soon as I unplug the printer from the USB port, voila, the mouse and keyboard start working again.
I have checked for IRQ conflicts and there aren't any.
All of the devices involved say "in working order" in the device manager, and all of the drivers are installed and working.
Even turning the computer off, and plugging everything in, and turning it on to let it "find new hardware" doesn't work.
I looked around in the CMOS/BIOS settings, and couldn't find anything amiss.
I probed around all over the place, and I'm stumped.
I'd appreciate any suggestions anyone may have that could help me fix this problem. Thanks, Phlegmox

 

Have a look under the device manager and under the USB hubs see how much power the printer is taking and find out what the mouse and keyboard require just in case the printer is taking too much power.

 

Thanks,
I'll look into that but I'm kind of doubting it will be the solution. Why would the USB port drain power from the PS2 ports? And why would it suddenly start that action when it had worked fine for a long, long, time?
I'm more inclined to think it's some kind of a setting that enables the USB ports when something is plugged into them, and simultaneously disables the PS2 ports. It's as if an option for priority to be given to the USB at the expense of the PS2 has been set somewhere in the system.
My dad did try to install the new mouse himself before I got back over there, and he may have done something he's not telling me about.
Thanks again. Phlegmox