are u sure, u have installed the correct driver?
if not, go to acer website or check ur included driver CD, and download/get the correct driver for correct windows version for correct laptop.
then install and test again.
also u can check in the device manager that ur webcam is working properly.
windows 7 does not contain build in software to show the webcam image, if this is the case, u can install 3rd party applications like VZO or cyberlink youCam.