install the PyUSB Windows installer from http://sourceforge.net/projects/pyusb/ (Make sure the Python versions match for the two installers. pyusb-0.4.1.win32-py2.5.exe, for instance, works with python-2.5.2.msi)
test it by running usbenum.py or accessing help for the usb module:
C:\Python25>python
[...]
>>> help('usb')
Help on module usb:
NAME
usb - USB access module
[...]