Guest account only cannot access admin Running Mavericks 10.9.5, iMac 12,1 21.5 Intel i5 2.7ghz 16gb.

I want to re-image the machine but it only boots with a Guest account. No Recovery partition and unknown admin password. 

Using "Install OS X Mavericks" but need admin password. It does have a Boot Camp partition that has Windows 10 which I have admin rights.  I want to get rid of everything but being guest on the Mac side without admin rights does me no good. 

My last thought was to just take the 1 terabyte drive out and erase it from another Mac then re install Mavericks OS.
I was able to download 10.9 installer from another machine and installed OS on 32gig flash drive. After booting to this 32gig flash, I Use disk utility from the flash drive running the macos and erase the Mac 1 Terabyte drive that I was unable to do anything with just Guest access. All is well. Can start fresh now.
Kenneth