Hi,
I ran Mach3 and two parallel ports on Windows7 for three plus years without problems. Did you find a copy of he optimization list....it had about 30-40m items on it?
Anyway you've got XP to work. If you were really concerned you'd ditch the parallel port altogether and get a genuine SmoothStepper or UC300 or 57CNC,
not some Chinese rubbish, then you could use just about any PC and Windows OS you care to name.
Craig