Hit F8 at boot up and choose "Disable driver signature enforcement".
Edit: Or to do it from within Vista on a permanent basis (above needs to be done on every boot when it's needed):
Click Start, then in the Search box type "cmd". You will see an icon for the command prompt appear in the Programs section.
Move your cursor over this icon, click and hold down your right mouse button on it, drag it off onto the desktop, release the right mouse button, and select "Create Shortcut Here".
Now right click this new shortcut and click "Run as administrator". You will now be presented with a DOS window and command prompt.
At the prompt, type the following: "bcdedit -set loadoptions \”DDISABLE_INTEGRITY_CHECKS” and press enter.
Notice there are 2 D's for disable, this is intentional.
Now reboot the PC and attempt to install the driver.