It isn't. If you have a Corrupt OS, factory resetting doesn't do the trick, because it just reverts to the OS files that are on the phone. I want to completely reinstall the OS using Microsofts files. I think I got it.
I did Vol-Up+Camera button on button and Zune asked me if I wanted to revert my OS. I don't know if this is going to put me on Mango or NoDo or the in between update. Will update.