From Stella:

As dated as this console may be, it's still VERY popular. Maybe it's the sense of nostalgia we all get when we flick the power on and see no more than sixteen colors at a time... maybe it's the blocky graphics or mono sound effects... or maybe, just maybe it's because the Atari system represented gaming simplicity, and made gaming fun. Whatever the reason, rejoice in knowing that you can play them all again right at your PC.

After download, you need to unzip the .zip file and run the Stella emulation runable file, then go through the ROMs directories which are alphabetic sorted and play your favorite Atari game.
For example supose that you want to play River Raid, so after run Stella emulation file go through Atari2600_Q-S ROM directory then find the River Raid file which is called Riveraid.bin here and enjoy the game.