How can I get an NES controller to work on my PC as a Joystick?
Solution 1:
RetroZone has a product which adds a NES port to your computer via USB. Also, you could buy an already converted NES controller which plugs directly into your USB port. (They also supply the same products for SNES.)
As for converting a controller yourself, Joystiq has a tutorial but it involves buying a kit from RetroZone.