To put it very simply, the BIOS is like an operating system for the hardware itself. It tells the various hardware how to interact with each other and the Operating System. It is responsible for selecting what device the computer should boot from, where video signals are sent and so on. If for some reason an update doesn't install properly the BIOS may not be able to do its job and boot the system from a CD, display video, etc. In this case the chip has to be removed and reprogrammed with an external device (since you can't boot into Windows and do it). This isn't precisely how it works, but I've found this explanation is easily understood by most. There is always a risk involved with firmware flashes that this may be the result. It isn't terribly common though and most manufacturers will happily correct the problem.