actually its model written on front is VGN-CR23G.
error message is "another app is using your cam. first close that app then use the cam."
That system did not come with Windows 7 it looks like, is that correct? There are no 7 drivers for the model, you can try using the Vista drivers. Webcams, sound cards and video cards are the things that have lots of issues if a non-supported operating system is installed, especially on a laptop.
Does the webcam show up in device manager with no issues? Is it showing in the Control Panel?