While I can't speak for "most", the limited experience I have had with IP cameras is that the stream coming off many of them is a bone-standard MJPEG stream. That is simply a stream of JPEG images, and any app that can interpret them should be fine. Microsoft has actually published a very small demo program, based on dotNet 4, that displays the output from a webcam.
Rosewill's webcam, by the way, uses a Java applet normally to show what's coming off the camera. I don't believe they use DirectX, or ActiveX, as the image output shows up fine on Firefox.