The SAM on an NT 4.0/5.0 box is kept in %SYSTEMROOT%\SYSTEM32\CONFIG as the filename SAM. %SYSTEMROOT% is a default of C:\WINNT.
By default, only Administrators have access to this folder.
An easier target is %SYSTEMROOT%\SYSTEM32\REPAIR as this is the backup copy of the SAM and other crucial /etc type files. By default that directory is open to everyone.
You can protect both by using a program called, SYSKEY. It'll enable 128bit encryption on the file making brute force take forever.
Profiles are kept in the %SYSTEMROOT%\Profiles directories but don't contain any NT password information, atleast by default.
Windows 95/98 boxes keep there passwords in .PWL files in the Windows directory. The name of the file is the name of the user. John is thus JOHN.PWL. They're are brute force crackers to attack those files, but if you have physical access you already 0wn the box.