This goes Beyond The Impossible
Here we have Super Mario Bros
running inside a NES emulator, running inside DOSBox, running inside a Windows XP virtual machine, running on a Windows 7 real PC.
And guess what?
The game is still playable!
Yes, even with all that emulation and virtualisation overhead, the game is still playable. If I hadn't tried it for myself, I wouldn't believe it was possible.
Quad-core Phenom II 955, 3.2GHz
GeForce GTX285 graphics card
Windows 7 x64