Pre-Vista it could be an issue if you didn't have debugging turned on via the boot.ini file and your system stopped booting.  Well in Vista and 2k8 you can enable "Debugging mode" on boot by hitting F8 after POST.  You can also turn off driver signing verification on x64 systems, so if you REALLY wanted to use that new graphics driver that's not signed you could.  Although you'd have to hit F8 on every boot and turn it off each time.

http://msdn2.microsoft.com/en-us/library/aa468633.aspx

Excerpt on one detail of importance:

Pressing F8 will cause the Troubleshooting and Advanced Startup Options menu to appear. One of the choices on this menu is Debugging Mode. If you select this option, you will be able to start Windows with kernel debugging support. The kernel debugger connection will be active at a baud rate of 19200 through the highest enumerated COM port (for example, COM2 if you have two ports).