Reset a (lost) Administrator password
Here are procedures for various Windows versions.
Windows NT/2000 (Server or Pro) - (non domain controllers!) Petter Nordahl's Offline NT Password & Registry Editor will reset the password quickly and easily. Read the instructions carefully! Your best bet is to set the password blank (using an asterisk), then change it to a real password after booting into Windows. Petter's tool can reset the password for any account on the system, but the most common use is to reset the Administrator password. I've done this one many times without problems.
Win7/Vista/XP Pro - Petter's disk works fine.
XP Home Edition. Use Petter's disk normally. When XP Home Edition boots up to the login screen, press ctrl-alt-del twice to get to the Administrator login. I think ... I haven't had to do this myself.
Windows 2000 domain controllers: Daniel Petri has written out a process that uses Petter's disk (above) and a little additional hacking. This process takes less than 10 minutes. I've done it 3 times without issue.
Windows 2008 - email me your experiences and links; I'll update this page with whatever works!