Attach your hard disk to another system, or if this system is multi boot than boot other OS, or boot with bootable cd or floppy which can understand NTFS. go to %systemroot%\repair of the OS which you have forgotten the password. You will get the file named (sam). Copy this sam file to %systemroot%\system32\config restart your pc & enjoy. there is no password.
Note: %systemroot% is an environment variable which correspond to installation location of your windows, typically c:\windows in Windows XP, 2003 and c:\winnt in 2000.