Why Is Microsoft-Supplied Driver "Not Digitally Signed" and Uninstallable?

I just bought a couple dozen Microsoft LifeCam VX-3000’s for our offices, but every time I try to install from either the CD in the package or with the latest download from Microsoft, I get a message that the driver is not digitally signed by Microsoft and to “try again later”. The install then aborts.

I HAVE gotten a couple to install after struggling with them for 30 minutes or so, but there doesn’t seem to be any rhyme or reason to when they do install and when they don’t. It just doesn’t make any sense that a driver supplied by Microsoft won’t install on a Microsoft system.