You will need to configure your video card to output 15Khz. The easiest way is to use Ultimarc's ArcadeVGA card. Also, you can use AdvanceMAME as well to do this, although not all cards output 15Khz.
In addition, you may need a video inverter. Nintendo games used "Negative Logic" video, which means that the video signals are dim at 5V and bright at 0V. Most other games, used 5V for bright, and 0V for dim. My table to this post (Scroll down) allow you to determine if your video is inverted:
http://www.arcadecontrols.org/yabbse/index.php?board=4;action=display;threadid=12425.
Note: If you find a homemade board with a 7404 chip, connected between the monitor and video cables, remove it and replace with an analog inverter. The 7404 and other digital inverters only allow a very limited color range of output, because they are digital.