Normally you are prompted to install drivers the first time you connect the phone to computer, but you may have dismissed the offer. For Windows 8, the driver installation fails.
Check whether the drivers are installed
At the phone, start the Settings app, then tap Applications, then Development. Check the box to enable USB debugging.
Now connect the phone to USB, and at the computer start Device Manager as follows, depending on your version of Windows:
Windows XP: Click Start, then Run, and type “devmgmt.msc” without quotes, and click OK.
Windows Vista/7: Click Start, then in the search box type "device manager " without quotes, and click on Device Manager at the top of the start menu.
Windows 8/10 (Desktop): Move the mouse to the lower left corner of the screen. Right-click the mouse, and then click Device Manager from the menu.
Now in Device Manager, check for the Motorola ADB Interface device as shown in the sample pictures below - when it appears under the Other Devices category, the drivers need to be installed. When it appears in the ADB Interface category at the top, or the Universal Serial Bus devices list at the bottom, the drivers are installed.
Drivers Not Installed
Drivers Installed (XP/Vista/7/8)
Drivers Installed (Windows 10)
Connect the phone to your computer via USB, and then proceed with the instructions below.
Windows XP / Vista / 7
Click Start, then click Computer. Look for a drive labeled “CDROM” that isn’t your CD or DVD drive. Double-click that drive, and/or execute setup.exe from that drive. Assuming that it does anything, it should start the installation program for Motorola drivers. Complete that with default answers.
Move the mouse to the lower left corner of the screen, right-click it, then click File Explorer. The Explorer window should be looking at “Computer” and the right-hand pane will show all drives connected to the computer. Look for a drive labeled “CDROM” , right-click it, and choose Open. There should be a folder called “Driver”, double-click it to open. Now there are two files shown with these names:
Double-click the 32-bit one if you have 32-bit Windows, the 64-bit one if you have 64-bit Windows. Complete the installation routine with default answers.
To confirm driver installation go back to Device Manager and verify that the Motorola ADB device now appears in the ADB Interface category.