Steps to reset a Windows account password given physical access to the box:

1. Boot a Linux live cd and mount C:somewhere.
2. Find magnifier.exe (somewhere in System32) and replace it with cmd.exe.
3. Boot to the Windows login screen.
4. Press WinKey + U
5. Start the magnifier, which should run cmd.exe.
6. Change the password in the Command Prompt.

Note that this will destroy all data encrypted with EFS.

I'm not sure who thought of this first, but if I knew, I would credit them here.