Disable USB pen drives

I have windows XP and Windows 7 workgroup with 15 avid workstations. I don't want that users use pen drives and external hd because virus and so on. To control i have disabled the usb ports from reading pen drives. To do that  simply make the permissions on the following files set to Deny:

C:\Windows\Inf\Usbstor.pnf
C:\Windows\Inf\Usbstor.inf

You need to go tools folder options then view and disable "Hide protected operating system files (Recomended)"

Pure and simple!