A registered executable that uses a basic serial. Have each downloader register and give them a slightly modified exe with its own md5 tied to their serial. If it doesn't match, have it work perfectly, but then after a week and a half have it disable itself and tell them to stop being a thieving pirate punk. Use your own time source so the crackers wont be able to just set their clock forward to see if their crack was successful.

The ship is taking on water and theres no saving this crew of misbehaving malfeasants.. unless they stop dealing in pirated digital goods. They will go under in Spain and Norway if they don't throw their pirated goods into the sea. Norway is extremely strict and the MPAA pressure against Spain will finally sink this disreputable crew of sailors under a torrent of lawsuits. Its a good day. Hahahel!