windows 7 ultimate 64 bit is not installing on my pc over windows 8
windows 7 not installing on my pc
You can't install over existing Windows installation. You have to remove Windows 8 first, or install Windows 7 as a "dual boot" configuration if there's sufficient space on the hard drive to create another partition.
Before you go ahead though, you should check that Windows 7 drivers are available for your PC model on the manufacturer's website. If there are none, don't install Windows 7 because chances are you'll have your work cut out trying to find compatible drivers for it.