Hi,
Although I should probably get a smacked wrist for this, my friends and I commonly swap games by doing this -
Whoever buys the game installs it on their machine - IE into
c:\latestgame\
Whever wants the game simply copies that entirefolder onto their machine. Invariably (and I mean every single game we've tried and there've been a fair few) when the new copy of the game is run it builds any required registry entries on the fly. Very occasionally you need to add a registry entryor two from the original machine - for instance one game stored the cd key in HKCU and nagged at startup if it couldn't find it. The only problems I've had was patching games, sometimes the patch looks for the installer subkeys which don't exist on the machine that the folder was copied to. In this situation the person owning the original simply applies the patch, and we recopy the folder to our machines. Getting rid of a game is as simple as deleting the folder and any related reg keys - just about always in HKLM\software.