Don't know for sure for PM, but the general policy for all kind of paid softwares seems to be that you can use one program in up to two computers, but not in both at the same time. Not to mention that any of these is very unlikely to work in modern OSs; PM has been dead for more tthan ten years now.
6, 6.5, and 7 can be used on multiple computers. For instance, for many years I had 6.5 on my main desktop, my backup desktop, and my laptop. No problems.