Did you install the drivers that came with your webcam? If you go into device manager, are there any devices with a question mark next to them? These are devices that need a driver installed.
Windows 7 requires signed drivers to prove they are from a legitimate vendor and have been tested to work with windows 7. If the drivers aren't signed, they won't install. Also, if you are using the 64 bit version of win 7, you will need the 64 bit drivers for your webcam.