Why did you name two posts the same thing? Yes, you can turn your webcam into a night vision camera by simply removing the IR filter in it, and adding a few IR LED's. Here is a site that explaines it well. http://www.hoagieshouse.com/IR/ If you don't have that type of webcam just skip to step 5.
Well most camera's can already see infrared light but they have an in built ir filter which filters out all the ir light. So by removing that filter the camera can detect ir light. It's not actually night vision because the ir led's light up objects in front of them with ir light that only the camera can see, but we can't see that light so that's why we call it night vision. So in fact the room is actually illuminated but we can't see it.