Instructions from:
http://laslow.net/2012/03/14/disable-driver-signature-enforcement-in-windows-8-cp/

So, one of the big issues I’ve had with the Windows 8 Consumer Preview is that Microsoft now not only forces you to use Digitally Signed Drivers (this isn’t new, as Windows 7 requires them as well), but also checks to see if the driver has been modified and will fail to install if it has.

This is a problem for anyone who needs to modify a driver .INF to support their device (*cough*Android ADB Drivers*cough*). Fortunately, there is a (slightly complicated) workaround.

To get started:

    From the Metro Start Screen, open Settings (move your mouse to the bottom-right-corner of the screen and wait for the pop-out bar to appear, then click the Gear icon).
    Click ‘More PC Settings’.
    Click ‘General’.
    Scroll down, and click ‘Restart now’ under ‘Advanced startup’.
    Wait a bit.
    Click ‘Troubleshoot’.
    Click ‘Advanced Options’
    Click ‘Windows Startup Settings’
    Click Restart.
    ???
    Profit!

When your computer restarts, select ‘Disable driver signature enforcement‘ from the list. You can now load your modified driver. Reboot again once the driver is installed and all will be well.




From:
http://arduino.cc/forum/index.php?topic=94651.0
http://arduino.cc/forum/index.php?action=printpage;topic=94651.0


Here is the process that I went through to load the Arduino device driver on Windows 8 Customer Beta x64

I had to create a catalog file for the INF and then sign the catalog file with a test certificate.
I then had to install the test certificate in the Windows 8 system so that it would “trust” the driver.

The attached files are only for the Arduino UNO. If you need files for other versions, I can create them.

This process is only for test purposes and this is not recommended as a permanent solution. If Windows 8 continues to enforce the driver signing requirement in the production version then the correct way to handle this will require Arduino to create .cat files for their drivers and sign them with their code-signing digital certificate.

Copy the arduino.inf, arduino.cat, and testcert.cer files to a folder on the Windows 8 system.
This is the procedure to install the test certificate in a system:
1.   Double-click the testcert.cer file.
2.   Click “Install Certificate” button.
3.   Select “Local Machine” radio button.
4.   Click Next button.
5.   Select “Place all certificates in the following store” radio button.
6.   Click “Browse” button.
7.   Select “Trusted Root Certificate Authorities”
8.   Click OK button.
9.   Click Next button.
10.   Click Finish button.
11.   Repeat the process starting at step 2, replace step 7 with “Trusted Publishers”, continue to step 10.
12.   Click OK button to close the certificate dialog.

At this point you should  be able to install the arduino.inf without getting any error.


List of CAs trusted for drivers:

http://msdn.microsoft.com/en-us/windows/hardware/gg487315

Certum Trusted Network
DigiCert - $223/year company, $129/year individual
Entrust.net - $269/year
GeoTrust - links to Verisign - $895
GlobalSign $229/year
GoDaddy - $199/year
NetLock Arany 
NetLock Platina
Starfield Root CA
StartCom CA
TC TrustCenter Class 2 CA II
Thawte Primary Root CA - $299/year
VeriSign Class 3 Public Primary CA
VeriSign Universal Root CA