Hey Kumardeep, hi,
plz tel me your windows 7 copy is genuine or pirated?
if pirated, i suggest u to take backup of all data from ur current windows 8 OS and perform a clean windows 7 ultimate install.
Once you have bought Win 7 Installation disk, change your boot order so that DVD drive is first. When you arrive at the 'Where do you want to install Windows' screen, use the 'Drive Options(Advanced) option to format the drive on which Windows 8 is installed. Choose that partition to clean install Windows 7.
Downgrade drivers for W7 here