How do I reset a late 2006 iMac

I was given a late 2006 (A1208) iMac. it boots to Windows XP by default. I’m using a windows keyboard and if I boot holding the alt key I can select the mac os. Problem is it boots to a user account and I don’t know the password to get it into an admin account. The user account does not have a password. I’m looking to do a factory reset to it but the user account won’t let me. Not being a Mac guy, I have no idea how to do this. I also do not have any disk for it but can get lion on a fat32 or exfat usb.

Been a while since had to mess with Snow Leopard and Lion, but some of the brain cells still hold memories. My recollection is having to boot from a provided Snow Leopard memory stick (which I think I still have), then upgrade to Lion through the App Store (believe Snow Leopard introduced the App Store). So, if you have Lion on the memory stick, and it is an install version, it is worth a try booting to it. Plug in memory stick, power on, and hold down alt key and see if the memory stick comes up as an option.