It is easy to lock a machine down, disable the floppy, and cdrom in the bios. Many times when you try to disable USB - it disables it entirely.
This can be a real pain on newer laptops or systems that don't even have a PS2 interface for the mouse or keyboard.
There is a simple registry change that will keep the USB storage drivers from starting when the system boots. Keeps people from walking up to a PC and copying data off with a USB key, but allows you to keep your scanner, keyboard, and mouse working.
As always - back your system up before messing around in the registry.
Just open regedit and browse to this key:
Notice the value 'Start'
Switch this value to 4, and USB storage devices are disabled.
Switch this value to 3, and USB storage devices are enabled.