M> As far as i know, this would not work alongside the normal
M> keyboard, as the OS only expects a single keyboard and/or
M> mouse on the sytem at any one time.

Xe did ask about "any OS", and for any OS that statement is simply
false. There are several operating systems that can perfectly happily
deal with multiple keyboards and mice. (The Linux kernel has had code
to deal with this for several versions, with the various
implementations, such as the GGI project, dating back to at least
1998.) Xe also asked about Windows XP specifically, and for Windows
XP that statement is simply false, too. Any laptop user with external
USB mice or USB numeric keypads will attest that Windows XP quite
happily deals with such things, merging all of the input event data
streams into one.