I consider a retro game to be any game that is part of the 5th generation of video games or earlier. Meaning that the gamecube, xbox, dreamcast, and playstation 1 games are off limits. The definition is most certainly up for debate, but try to keep this thread away from modern games (no call of duty please).
Running old games:
Running retro games can range from very simple to very hard. I consider myself to be more or less proficient in the use of DOSBOX (an expert by no means), however, I know very little about other emulators and can't proved any help. Here is a very brief tutorial on running old dos/windows games on modern systems.
I suggest DOSBOX: http://www.dosbox.com/
Create a file on your hard drive in a location other than program files (in windows vista and above) called oldgames.
Unzip/install dosbox into that folder preferably in its own file (C:/oldgames/Dosbox)
linux users can just install the pre-packeaged repository (sudo apt-get install dosbox) or a variation of that depending on your distro. mac users I don't know.
Running windows games:
vitrual apple ][ website
Running commodore games:
http://c64s.com/
Some sort of emulator...
Personal favorite site for emulators:
http://www.emulator-zone.com/
Enjoy