Don't ask me, Mine's part of an MX Duo and it's had this issue with both my P4 and my SN41G-2 Shuttle cube.
If I knew why it did it I'd have fixed it. It's been like this even with my Abit IS7 board so I really doubt my USB drivers are suspect.
Phenom II 940 Black Edition quad core @ 3.5Ghz
2x1 gig OCZ PC26400 Platinum, 2x1gig GSkill PC26400
Upcoming EVGA GTX470
Some other stuff,
Even more stuff,
If the automobile had followed the same development cycle as the computer, a Rolls-Royce would today cost $100, get a million miles per gallon, and explode once a year, killing everyone inside. --Robert X. Cringely, InfoWorld magazine