I've got a USB mouse (MS) that came with a USB to PS/2 converter. It works fine hooked up to a PS/2 port. I run it through a KVM switch that is PS/2 and not USB so this is necessary. One of the boxen ...
Add or Remove Programs entry for FTDI USB Serial Converter Drivers. This is a driver pack that allows USB ports to be used to connect to Serial bus devices with a USB - Serial adapter cable.