cheezhead, you could (theoretically) open up your PC case, and take a look. As you can see from the picture of that converter, the connectors to the motherboard are quite different.
All you need is some comparison material.
<img src="http://www.3dxtreme.net/images/reviews/Aerocool%20AeroPower%20II+/pic8.jpg">
That's a standard ATX connector. On the left, there's an AUX connector, which is usually not needed. In the middle is the bog-standard ATX motherboard connector (which you need to check!), and on the right, we have the extra connector for Pentium 4 motherboards.