Just had to reset a lost Windows XP password for the second time and thought it would be useful for myself to have a howto online:

1. Boot from the System Rescue CD
2. Mount the disk:
   ntfs-3g /dev/sda1 /mnt/windows -o force
3. This depends on the OS install if there are caps used, but usually:
   cd /mnt/windows/Windows/System32/config
4. List the account in the SAM datbase:
   chntpw -l SAM
5. Reset the password of the Administrator:
   chntpw -u Administrator SAM
6. Press 1 to clear the current password
7. Reboot the server/workstation:
   reboot

So if you’ve forgotten your Windows XP / 2000 / 2003 password this is the way to clear it or reset it.