Here's a neat little tool, which will allow you to boot to almost any Windows machine even if password protected. It does not in any way change any passwords on the host machine - rather, it patches some Windows Logon Authentication files during boot-up in RAM, so no files are affected on the hard drive. It leaves no trace of having been used.
It has options to boot from a USB, CD, or floppy.
Read more here:
http://www.piotrbania.com/all/kon-boot/index2.html

If this sounds useful, feel free to PM for the link.
So, assume you have installed this app onto a very cheap tiny thumb-drive. Making sure the computer will boot from USB, simply reboot machine with thumb-drive in port, and when presented with the logon screen, enter nothing, or any gobbledygook, and voila! - continue to a normally functioning machine.
Note: I have not tested this as I normally do, as I have set up my machines to have no logon screen. Worked perfectly though on a professional webcast show.