Here is the NES Interface schematic. All diodes are standard signal (1N914) diodes:
 

Multiple NES pads are supported using the same interface as SNESKEY. For pads 2..5 hook them up the same as pad 1, but don't make the DB25-10 connection. Instead, make the connection according to the following table:

NES PAD      DB25 PIN
    2            12
    3            13
    4            15
    5            11

Here is the software to run the controller on your computer.

 Direct Pad Pro 5.0
 

Once you've got the interface built, it's time to install the drivers so that Windows can communicate with it. Go to the "Control Panels" window, and select the "Game Controllers" icon.
Select the "Add..." button in the Game Controllers window, then select "Add Other..." and "Have Disk..." in the windows that appear. Specify the path into which you've extracted the .ZIP file, and hit OK.

Select "DirectPad Pro Controller" from the list (this version does not have individual drivers for each joystick type). You will then be returned to the "Add Game Controller" dialog. For the second time, select the "DirectPad Pro Controller" and you're almost done. If you're running a Dual-Shock controller and want force feedback, select the "DirectPad Pro Force Feedback Controller" from the listing, after performing the "Have Disk" operation.

Finally, double-click on the newly installed joystick. Use the dialogs to configure to the proper interface, parallel port, and controller ID (when using multiple controllers).

If you're using multiple controllers you need to repeat this exercise for as many pads as you have connected.