Would like to play Atari games on my Win XT, but can't get the emulator to run.?

Have tried unloading and reloading the emulator to no avail. I want to play games that were designed for the Atari Computer system, NOT the 2600 system that ran off your tv. I had an Atari Computer...wish I kept it now. I have a Windows XT. Am I trying to do the impossible?