How do I force the install of OS x over the currently installed version?

I have a 2007 Mac Pro 2,1 I purchased surplus from a college. Upon start up it goes to user/admin. log in window. There's no way to recover the password from the college. So, I have purchased from Apple a Snow Leopard install disc. I have inserted the disc into the optical drive and shut down the computer. My hope was that upon start up it would start installation. The fact is that basically nothing happens. Computer still goes to the same login screen. How can I force it to start an overwrite install? Thanks in advance!