If the two (cam + laptop) have not worked together before, it might be due to drivers problems. Most drivers are very OS specific. In some instances Vista has been known to have USB and/or compatibility issues. On the otherhand XP sometimes requires that you uninstall the old drivers, reboot before (re)installing new drivers, reboot before installing the hardware and allow autodetect and auto install.
If the two (cam + laptop) have worked together before and now doesn't, it might be due to hardware problems such as the cord, the cam itself or the USB port of the laptop. If your other USB devices works, then USB port(s) can be ruled out. You can verify that the camera works buy installing it in another PC.
You can verify OS compatibility of your camera as well as updated drivers at the Logitech website
. Additionally, you can check for Microsoft updates/patches and fixes like this USB patch
Should you need additional information, pls post back with your cam's brand/model, your laptop's brand/model and your OS version.
Hope this be of help. Good luck and kind regards.