Advise you to always search Codeproject.com first. Accidentally, there is a good article on this very topic - Simple URB (USB Request Block) Monitor
But there is even better sample "USB Device Viewer" in Windows DDK (in C:\WinDDK\7600.16385.1\src\usb\usbview). But it does not constantly "monitor" ports (that functionality you could, probably, add by yourself).
P.S. I was wrong, it has option "Auto Refresh".