reset muliple users passwords

How can I reset the passwords for multiple users, all at once? Rather than resetting each user's password individually. i was able to do this through the active directory but since we mirged all my other domians as one this summer and my network administrator quit and left town didnt finish the job i finished most of it but now i cant do this no more. I could always do it before on the individal domains but since merged all the domains to one that functions is gone. I think Micrsoft has add on or a .dll download file somebody told me but i cant find it yet
i have 5 DC and 1 P.C. all have windows 2003