Hi. If you bough an new Arduino but you receive "Unknown USB device(Device Descriptor Request Failed)" then this tutorial is for you. And also sorry for slovak language in images.
So let's start!
Step 1: Open Device Manager
In Windows 8 or 8.1 press "Start logo + X" and select Device manager (Správca zariadení).
Step 2: Find Device
Find device that says "Unknown USB device(Device Descriptor Request Failed)" and go to next step. You can also receive Unknown device (Neznáme zariadenie). So next step ahead..
Step 3: Update It!
Right-click and update device's driver.
Step 4: Manually on It
Hit 2nd option (Find driver manually).
Step 5: Again...
Press find manually
Step 6: Select Type
Select "COM Ports" or similar
Step 7: From HDD
Hit from HDD and then choose path to folder. Default is "C:\Program Files (x86)\Arduino\drivers" on 64-bit.
On 32-bit choose "C:\Program Files\Arduino\drivers"
If you have Arduino Nano or similar with FTDI chip (NOT an ATMega), you need to try look in the folder \drivers\FTDI.
Step 8: Choose the Arduino
Now if you need it uncheck "Show only compatibile hardware" or similar or just choose your Arduino and click "Next".
Step 9: Finally...
Your Arduino should be successfully installed to your system. So Close it, Read your port aaaand... SUCCESS!
Thanks for reading.
Arduino ID: 0xN35T