Can i use a digital camera or a camcordr as a webcam instead of a normal one? and how to install and use it?

You can, if you have video input card (some have a TV turner built in). You can use anything that has a TV out (a/v out plug). Follow the instructions, there is USB version and it is easy to use your digital camera (a/v out) or a video camera, security camera, even a VCR if that is what you want to do.
Some camera is all ready to be use as a web camera.

Can't really tell you how to install and use because it varies depending on the card and/or camera.