I had to reset the password of an account on a system running windows 7. There is a wonderfull linux live CD to do the job really easy. Go to http://www.sysresccd.org/Download and download the according ISO image. Burn it and boot your windows machine.
After sysresccd has booted mount your windows partition:
ntfs-3g /dev/sda1 /mnt/windows
Change directory:
cd /mnt/windows/Windows/System32/config
In there you should find a file named SAM. List users by running:
chntpw -l SAM
Clear password:
chntpw -u SAM
Chose 1 to clear the users password. Write this changes to disk by pressing Y.
Taddaaa. Reboot. Easy as can be.
I presume that you do have some basic linux know-how. e.g. how to find partition types. If you do have problems doing this leave a comment.