On my G74SX-A1, in Device Manager, the "USB Composite Device" under "Universal Serial Bus controllers" has a "Location information" property of value "Port_#0002.Hub_#0003" in the "Details" tab of Properties for the USB Composite Device. The "Bus reported device description" property has a value "ASUS USB2.0 Webcam". This makes sense, since the Webcam/Microphone is a composite (serving more than one purpose) device.

So I believe it is your Webcam that is randomly disconnecting. For a first step in troubleshooting, I would uninstall the ASUS USB2.0 Webcam in Device Manager, then reboot the system. It should detect the Webcam on startup and install the Windows 7 Plug N Play driver (which is currently running on my Webcam).

If that fails to resolve the issue, I would boot into Safe Mode and install the latest Intel Chipset driver from the command line using the -overall switch. This driver may be downloaded from a link in the OP of http://rog.asus.com/forum/showthread.php?5224-Drivers-Apps-and-How-to-s. Instructions for installing using the -overall switch are at the bottom of this post.

If you choose "Action - Scan for hardware changes" in the top menu of Device Manager, does "ASUS USB2.0 WebCam" appear under "Imaging Devices"? If not, I think it is likely you are experiencing a hardware problem. I remember a post from late lasy year where the user could get the camera to connect by wiggling the laptop lid - definitely a hardware problem in that case.

How long have you had your G74?

That makes it sound more like a hardware problem, since it is no longer detected by Windows.

Are you still within your retailer's exchange/return policy? If so, I recommend exchanging it for a new unit, or returning it for a refund.

EDIT: Check out this post to see if the camera is disabled in Power4Gear or Windows Advanced Power Management settings: http://rog.asus.com/forum/showthread.php?4218-Optical-Drive-is-Gone&p=29045&viewfull=1#post29045