Without physical alteration, the original Xbox console cannot play any games besides those designed for the original Xbox system. Hardware components of the Xbox are not powerful enough to render properly software made for the Xbox 360.
Many modern systems have backwards compatibility, which means that the newer system can play software from the older system. The Xbox 360 can play 400 different software titles from the original Xbox system. Due to hardware differences between generations of consoles, no existing console can play games designed specifically for a newer system. Some downloaded software, such as games found on the Xbox Live service, may be accessible on multiple iterations of hardware.