Acer aspire 5742 web camera software

First of all check in Device Manager that the webcam driver is there and has no errors.
If it's not there or has a question-mark, download the driver from here:
http://support.acer.com/us/en/product/default.aspx?tab=...

There are three different camera drivers listed there but you only need one of them.
Go to the top of the drivers list and click "Download Hardware Vendor Detection Utility"
Run the utility to reveal the webcam vendors name, then download the webcam driver which
has that vendors name.

Install the driver when it's finished downloading.