Easy (and scary) fix.
Boot from Windows 7 DVD and get to a command prompt. Find ACCWIZ.EXE and CMD.exe on the local hard disk. Make a backup copy of ACCWIZ.EXE and CMD.exe. Rename CMD.exe to ACCWIZ.EXE.
When you reboot the machine, click the Accessibility tools at the login screen. A system privledged command prompt will show up.
Use "Net user" to change the password.
You will need to reverse the earlier steps once you get in. I've hacked my way into my own servers using this method when an admin password change (with 20+ random characters) went poorly. This works when Rainbow Tables won't.