****Disclaimer*****
HP doesn't recommend the installation of Windows 7 on a consumer computer shipped with Windows 8, nor will it support the installation of Windows 7. HP will not provide Windows 7 drivers and suitable drivers may not be available for the hardware in the computer. Also, Windows 7 may not support some of the hardware or software features designed for the computer.
Make sure you create your HP Recovery Discs or USB Flash Recovery media before modifying or changing ANYTHING.
You will need to DISABLE "Secure Boot" and maybe ENABLE "Legacy Support" in the "BIOS Secure Boot cofiguration" on the settings page similar to what is shown below, before you can attempt to install Windows 7;
or
Additionally, as the hard drive is most likely partitioned as GPT, you may have to boot the computer using the "UEFI - CD/DVD drive" or "UEFI - USB Flash drive" options in the "F9" boot menu. If "UEFI - CD/DVD drive", "UEFI - USB Flash drive" or something to that effect doesn't exist in the boot menu, please post a screen shot and/or descibe the listed boot options.
I suggest using Windows 7 SP1 for the best intallation experience. If you need Windows SP1, please see "How to Install Windows 7 Without the Disc" to download and create your own Windows 7 SP1 disc. If the DVD refuses to boot as UEFI, you may need to use a USB Flash drive (may require DISABLING "Fast Boot" in the BIOS) as the installation media. Please download the Windows 7 USB/DVD download tool to create a Windows 7 SP1 USB Flash drive. Please note that some computers may not recognize a valid UEFI enabled USB Flash drive if it is formated as NTFS and may require a FAT32 formatted USB Flash drive. You should be able to make a FAT32 formatted Windows 7 installation USB Flash drive using Rufus, UNetbootin, Universal USB Installer, or WiNToBootic. I like Rufus, as it has options for "GPT partition scheme for UEFI computer" and "FAT32".
Most of the Windows 7 64-bit drivers from the HP Pavilion g7z-2100 CTO Notebook PC Drivers page should work on your computer. Please post the Windows Device Manager's "Hardware Ids" for all remaining missing or unknown devices, as shown in the example below;
If you have any further questions, please don't hesitate to ask.
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