XP pro runs it fine - just turn off the cutesie stuff if you are limited on memory. When you first install P5 [to bypass the serial registration] - before running P5 program - run the updates and then you don't have to do the online register thing. I put together new machine and had bad motherboard so ended up installing XPpro 4 times and had to call microsoft to install OS but when I reinstalled for fourth time the P5, just updated with downloaded SR2.1 and wasn't required to do online register.