Microsoft discourages installing device drivers have not been specifically tested and "signed" to work with Windows. To that end, Windows XP, Vista and 7 have driver signature enforcement policies that prevent the installation of "unsigned" drivers. In Windows XP, you can easily circumvent this protection by clicking "Install unsigned drivers," but that option has been eliminated in Windows Vista and 7. Several registry or policy hacks have been released, which are now reportedly ineffective or inconsistent. Only one method is still verified to work without installing third-party hacks.
Restart your computer.
Press the F8 key continuously as Windows boots up. This will bring up the "Advanced Boot Options" screen.
Select "Disable Driver Signature Enforcement" using the arrow keys and press "Enter." Windows will no longer try to verify driver signatures, and you will be able to install unsigned drivers. You will have to set this option every time you restart your computer and wish to run the unsigned device driver.