I count only retail games, not old games from other digital stores like DotEmu, GamersGate or Steam:
System Shock 2. I installed the retail version maybe late last year with some community patches, but a bit later uninstalled it and installed the GOG version instead. The retail version seemed to work fine, but I decided to still go with the GOG version, just so that if I have any issues, I have support from GOG.
Before that, I think it was Heavy Gear, about two years ago? I used my old laptop (IBM ThinkPad T41) running Win98SE for it as the newest patch for HG seemed to have some issues in both Windows 7 and even XP. It ran fine on that laptop, finished the game there.
Post edited July 15, 2014 by timppu