Reset password in WinXP or Vista

Boot from Linux CD with ntchpw utility installed. Most of the time I used PING. Pretty sure most of Live CD’s do have that utility.

Mount Windows partition to Linux:
ntfs-3g /dev/hda1 /mnt/ntfs
mount -t ntfs-3g /dev/hda1 /mnt/ntfs -o force
The last one is used if windows did not shut down properly.
In Windows partition Navigate to WINDOWS/system32/config
Execute chntpw SAM. As a safety measure you can copy SAM file somewhere available to restore, if things go wrong.
This utility will ask you number of options. Blank the password, do not set a new one. Agree to write hive files when finish.
Restart computer to boot into Windows.

By default chntpw is loading administrator account. You can load another user if required. Read man page for chntpw to figure out how to do this.