As Chris Ortner pointed out, the Kinect for Xbox sensor is compatible with Windows 7 for development purposes only - you need to have the SDK installed.
It might be worth checking to see if the SDK has installed properly.
In Device Manager you should see if it has the following:
- Microsoft.Kinect
- Microsoft Kinect Audio Array Control
- Microsoft Kinect Camera
- Microsoft Kinect Security Control
And:
- Sound, video and game controllers
I have read that it is recommended that you do not have the sensor connected when installing the SDK. This is also implied by the msdn article setting up a kinect sensor. I have also read, although unable to provide a reference, having things like OpenNI or libfreenect may cause issues and should be uninstalled before installing the SDK. The link also provides other troubleshooting tips.
One last thing to check, ensure you are plugged into a USB 2.0 or USB 3.0 bus. It is one of the hardware requirements, and I am aware that one of the KinectSensor states is InsufficientBandwidth.