To install the Android USB driver on Windows 7 for your device:
1. Connect your Android-powered device to your computer's USB port. 2. Right-click on Computer from your desktop or Windows Explorer, and select Manage. 3. Select Devices in the left pane. 4. Locate and expand Other device in the right pane. 5. Right-click the device name (such as Nexus S) and select Update Driver Software. This will launch the Hardware Update Wizard. 6. Select Browse my computer for driver software and click Next. 7. Click Browse and locate the USB driver folder. (The Google USB Driver is located in \extras\google\usb_driver.) 8. Click Next to install the driver.

then execute the following commands in your cmd
1.adb kill-server
2.adb start-server
3.adb devices
Hope it Works friend :)

To install the Android USB driver on Windows 7 for the first time:
1. Connect your Android-powered device to your computer's USB port. 2. Right-click on Computer from your desktop or Windows Explorer, and select Manage. 3. Select Devices in the left pane. 4. Locate and expand Other device in the right pane. 5. Right-click the device name (such as Nexus S) and select Update Driver Software. This will launch the Hardware Update Wizard. 6. Select Browse my computer for driver software and click Next. 7. Click Browse and locate the USB driver folder. (The Google USB Driver is located in <sdk>\extras\google\usb_driver\.) 8. Click Next to install the driver. Or, to upgrade an existing Android USB driver on Windows 7 with the new driver: