I was installing a program and it had this thing called newsid, i stupidly did it, and then i restarted my computer and my administrator account was is now a guest account and it won't let me change it back. I can't access my old files because it says no permissions.

Hi Martin, on Welcome Screen, please choose "Other User" and type "Administrator" with password and see if you can log on the computer. If so, please then configure the user or recreate another administrator account. Hope this helps!Sean Zhu - MSFT