I have a question about a Dell Computer?

My dad got a Dell Optiplex 780 from a friend, and on the tag it says it is supposed to be Windows 7 Operated, but for some reason it has Windows XP on it, and it refuses to work with the Windows XP. He cant factory reset it to get it to work and has been trying to get Windows 7 back on it but no luck since we... show more My dad got a Dell Optiplex 780 from a friend, and on the tag it says it is supposed to be Windows 7 Operated, but for some reason it has Windows XP on it, and it refuses to work with the Windows XP. He cant factory reset it to get it to work and has been trying to get Windows 7 back on it but no luck since we don't have a disk or a USB that has a bootable version of it on it. So what can he do to fix this Dell Computer?
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