1) Download the software BART-PE. With it, you can create a so-called Preinstalled Envrionment.
1a) optionally download a created Image via torrent (Isohunt). But be aware, this may be illegal in some countries.
2) Boot the cd on the Machine, where you forgot the pw.
3) After BartPE has started, press Go.
4) In Programs, select Password Renew -> select target.
5) Select the windows Partition.
6) Enter new password
7) Reboot.