Here is simple solution for Windows 7 and Nexus 5 on Android 5.
- Download the Nexus 5 Drivers from http://androidhost.org/jelry
- Extract the zip contents and place all files in a single folder on your desktop.
- Connect your device to your computer.
- Launch the Device Manager on your PC.
- Now you should see the Nexus 5 listed in the hardware list.
- Right-click the ‘Nexus 5′ line and then click on Update Driver Software.
- Next, click the ‘browse my computer’ option.
- In the new window click on ‘Browse…’ button.
- Go to folder unzipped at step 2. Select the folder where you extract the USB Drivers. Click Next. – make sure to tick the subfolder box too.
- Now, the Windows installer will search for Nexus 5 drivers, click Install when asked for permission.
- Wait for the process to complete and then check the Device Manager list to confirm that the installation was successful.
Original: http://www.android.gs/download-and-install-google-nexus-5-usb-drivers-adb-fastboot/
Note: do not forget to enable USB debugging on your device :)