For anyone having the driver signing issue on Windows 8, the
bcdedit answer above doesn't seem to work because Windows 8 has some sort of new Secure Boot policy that prevents running that command successfully, even as admin (on my machine, at least).
What worked for me was running
shutdown /r /o /t 00 as admin, then selecting
Advanced options, then
Startup Settings. Click the
Restart button to restart and enter the
Startup Settings menu, then select option #7 (
Disable driver signature enforcement). After restarting, Windows should now allow installation of the
com0com drivers (though it shows big warning messages that the drivers are untrusted).
(I found the details on getting into the advanced startup settings here.)