GameSpot allows you to keep track of the games you have for every system and for PC. I haven't done this for PC games but I have done it for many of my others. I try to make sure I do this when I buy a new one, but tend to forget about it sometimes. ;) Anyone that is interested in mine can check it here:
http://www.gamespot.com/users/Saurgrist/games
If you sign up for GameSpot and create your own, post it here. It's easy to do, since you search for the game and on the results, click "Add to My Games." And that's all you have to do.