Podívej se nejdříve do správce hardwaru zda Window správně nainstaloval Arduino. Pokud ne zkus tento návod:
1. Windows Key + R
2. Enter shutdown.exe /r /o /f /t 00
3. Click the "OK" button
4. System will restart to a "Choose an option" screen
5. Select "Troubleshoot" from "Choose an option" screen
6. Select "Advanced options" from "Troubleshoot" screen
7. Select "Windows Startup Settings" from "Advanced options" screen
8. Click "Restart" button
9. System will restart to "Advanced Boot Options" screen
10. Select "Disable Driver Signature Enforcement"
11. Once the system starts, install the Arduino drivers as you would on Windows 7

Steps to install driver after Driver Signature Enforcement has been disabled:
1. Right click the Unknown Device in Device Manager
2. Select Update Driver Software
3. Select Browse my computer for software
4. Click Browse button
5. Select the Drivers folder under the location of your Arduino software installation. (Do not select the FTDI folder inside the Drivers folder)
6. Click OK
7. Click Next

Update:
If you have a new system that came with Win 8 pre-installed, Secure Boot has probably been enabled in the BIOS.
The steps above, probably will not work unless Secure Boot has been disabled in the BIOS before performing these steps.