For whatever reason, Microsoft has decided that driver signing is now REQUIRED in Windows Vista x64 (and I hear this also applied to Windows 2008).  There used to be a couple of different ways you could disable this requirement, but as of Vista SP1 neither of these methods work anymore.  There are also individual pre-SP1 hotfixes that “fix” Windows so you can’t disable the driver signing requirement.  This is a big issue for anyone who wants to use tweaked video drivers, or for anyone who wants to use VMWare Server (which still uses unsigned drivers for whatever reason).  The only way I’ve found to disable the driver signing requirement is to hit F8 on boot and choose the option “Disable Driver Signature Enforcement”.  This has to be done at every boot.  :/  Fortunately I’m not the only one who’s run into this problem.  Someone has written a little program called ReadyDriver Plus, which gets around this issue by stuffing the keyboard buffer at boot time, so it selects the “Disable Driver Signature Enforcement” option from the Windows boot menu for you.  ;)  It works perfectly fine for me.  Just installed it, selected the default options, rebooted, installed VMWare Server, and everything works just fine.