Is it possible to play multiple instances of the same game on one computer

It is, well for most games but there might be exceptions

To do so you must find your game folder (Like C:/ Program file/Microsoft Games/ Age of Empire, or whatever is your game) andthe you copy the entire folder in another folder in the same root folder. So if you wanna copy age of empire you copey the entire content of the "C:/ Program file/Microsoft Games/ Age of Empire" folder into a folder like "C:/ Program file/Microsoft Games/ Age of Empire Copy" and then you have a second instance of age of empire, to start that new instance just double click the .exe file that starts the game (usually in the "C:/ Program file/Microsoft Games/ Age of Empire Copy" folder"

Don't know if it's clear but i did my best (English isn't being my first language :D)