The guide is likely outdated; the drivers are certainly for Windows 8.1 - the verification process was completed (as noted on the download page). Did you follow the ** special instructions for Windows 8/8.1 installation**?
What is the computer's make / model / spec?
Are you connecting the CDJs directly via the USB or are you using a hub? Are the ports USB2 or USB3?
Which version of Serato DJ are you running?
What is the firmware version on the CDJ?