Just setup up my Powerbook/XP system using VNC (an open source remote control protocol) allowin gme to control my XP machine via a window on my powerbook. This means that the powerbook can now span across two displays (Cubase arrange screen on 17" is way better than 12"!) whilst still having full control over the PC. Hurrah.
To do this required setting up a simple ethernet connection between the two machines and installing the VNC software - I'm happy to post a tutorial if anyone out there would find this useful.