Turning an old digital point and shoot camera into a webcam
There may be a hardware solution to your problem, but I would generally consider it too much work when a webcam can be commonly bought retail for less than $15.

The concept of "Tethering" allows control of the camera to actually take pictures etc as though you are manually using the camera. It has to be supported in the cameras firmware. Then a PC application can be used to control the camera. 

Otherwise look for drivers that allow the camera to appear as a WIA device (similar to a Scanner) on your computer. Then use a photo application or even MS Paint to view the active camera view and capture screen shots or video as the application allows.