Restore Windows 7 from windows vista? -

this might be long but please I need help really BAD!!! Ok so i installed windows 7 on my computer that has vista. but i wanted vista back but i couldnt find the disk (windows vista home premium) so i borrowed a Windows vista ultimate disk form a friend and installed it but it installed over the Windows 7 and i can t get it back.(It renamed the Windows 7 files to windows.old) When i choose earlier version of windows it goes to xp (which i dualbooted vista homepremium and windows xp). All i want is to get back Windows 7 Please please help

What is windows 7?