If you are planning to downgrade your PC from windows 8 to windows 7, sometime the hard drive doesn’t detect at the time of installing windows 7. In this case, you can try following methods.
Go to bios setting (press f2 or del or esc, depends upon brand)
find sata mode or Boot mode, change to AHCI or CSM Boot for some model you need to disable secure boot first before changing Boot mode.
make first boot device as CD ROM
Save, exit and reboot
If still cannot detect hard drive. Disable intel rapid storage technology from bios and try.
If again cannot detect the hard drive, you have to load the sata driver at the time of installing windows 7. Click Here to download the driver.
Make sure you note down all drivers name of PC before formatting (eg. LAN, Wireless LAN, Audio, Graphics etc…)