The Mach 3 you download is identical to the one you buy, and will operate a fully functioning CNC system. I am still using the downloaded version. There are restrictions, of course, for example, only 500 lines of code at a time can be run, and I think there are other shortcomings with threading. All that happens when you buy a licence is that these restrictions are taken off. I don't think you even have to diisturbe the downloaded version.
Download Mach 3 with the driver and install in as per the instructions. There is no restriction in XP and if the driver installs properly it should work fully. I would use the latest lockdown version.
I did come across a couple of weeks ago a restriction in the Bios of my particular computer, where the LPT1 port had for some reason, become disabled. I switched it back on and everything was fine again. I would try the computer with an old fashioned LPT1 printer (if anyone still has one). If the printer works, there is no reason why Mach3 will not.
The leaning at the moment is towards a Smooth Stepper, which has a USB connection to your PC, and controls your machine. I suppose this brings us into the modern era - but - no - the old system still does work.