Computer emulators of game consoles is a legally tricky area. The generally accepted rule of thumb is that if you own the cartridge, you can play the game on your computer. When Connectix came out with a Sony Playstation emulator that required the Playstation CD, Sony challenged that and maintained that the game can be purchased and played with the implicit understanding that you own a console. A legal battle ensued and continued until Sony threw in the towel (while they were ahead, even) and entered an agreement with Connectix. Here's an article explaining the details of the Sony/Connectix battle.