Empires Dawn of the Modern World, it's an older RTS game, made in 2003. It's not free but it's probably quite cheap for being so old. It would definitely run on almost any machine, and it stills looks pretty good for being so old. Sadly it doesn't have any online multiplayer since they shut down the Gamespy server a few years back, but you can still play with friends but they would have to connect to you through your IP.