generally it should still function.
do you happen to have anything other than Mouse/KB plugged into your USB or front port connections?
I randomly get that too, it asks to plug it in to a higher speed port, but still functions and works the same.
When I looked at AppleNewton, he does have PM and e-mail enabled.All I see under contact information is:
"AppleNewton has no contact information."
In fact, that's all I see for any user, nor do I even see an option to switch on PM for myself...baffled.
Chances are that it is using the stock Windows drivers which don't understand the ICH in the Mac Pro (which controls the USB ports), so it's using USB 1.1. speeds. if you install Boot Camp's drivers - or the drivers from Intel's site for a 5000-series chipset - you should see USB 2.0 speeds and support.