I don't know about Paint Shop, but Photoshop comes with a serial number that is registered to the original buyer. If that buyer wants to sell their license then it must be done through Adobe. Most programs are like that. This is how the people that write software get paid for their work.
If you want a powerful free image program then check out
Gimp. There is a Windows port, but I'm not sure how good it is. It is a very good program under *nix, but I do still prefer Photoshop, but then again Photoshop is $600.