In Windows 7 (and probably on Windows Vista too), all the drivers must be signed. If not, the OS will refuse to install them. I think this is a good feature and it probably gets rid of 90% of the stability problems of Windows.
However, today I need to install an unsigned driver. I sometimes use com0com to test the serial communications code of one of my applications. This tool allows you to create virtual COM ports that are connected. Very useful.
However, installing the new ports requires to use their own unsigned drivers, and Windows 7 doesn’t allow that. In order to bypass this limitation, you’ll need to reboot your computer and press F8 to get to the Advanced Boot Options menu. There’s a boot option at the bottom to Disable Driver Signature Enforcement. This will allow you to install the ports just fine.