How to sniff a USB port under Windows?
After five years waiting, now it's possible to sniff usb packets on windows
See http://desowin.org/usbpcap/tour.html for a quick tour. It works pretty well
Since people don't seem to realize it, Wireshark does monitor USB traffic and has a parser for it; but the catch is it only works under Linux. Wireshark on Windows will not do this.
It may be possible to plug the USB device you want to monitor, along with a Linux machine (with Wireshark running) and your Windows machine and just use the USB device under Windows.
Problem with the above? I don't know how the Linux machine or the Windows machine will detect each other.