How can I connect my Digi Camera to my laptop and use it as a webcam ?

Digital camera : Sony DSC W-50
Laptop : Dell Latitude D410.

Unless your camera came with the software to allow this then it can not happen.
Check on the camera website and see if it is possible, download the software (is there is such a thing), something tells me this will not happen.

with a usb cable

i doubt u can do this.The software that comes with the camera allows only photo upload & downloads through a USB cable

I do not think that action can be done

This camera SONY DSC W50, (all W series) cannot be used as a Webcam because it doesn't support video strearming to PC.

Very few good cameras have this feature, last one i used was FUjifine pic S5600 6 Mpix, 10x Optical zoom.

Sony DSC cameras don't offer this. Perhaps the MiniDV handycam do have this option..