Ugh, don't get me started on Win 8... If I had a gun with two bullets and was in a room with Windows 8, Hitler, and Osama, I'd shoot Windows 8 twice...
Went to install Win 7 on my sisters laptop that came with Win 8 and the damn thing wouldn't let me. Something about the way Win 8 is coded or something?
Problem is manufacturers are setting up their bios (basic input/output system, the motherboards system Independent of the OS) to be more streamlined with w8 for faster boot times. In fact, when you shut down w8, it doesn't really shut down. It goes into a form of hibernation.
Trick to getting win7 on them is to force shut down w8(hold the power button until it turns off, or yank the battery/cord) (this is important so that w8 shuts all the way down, not hibernate, allowing you access to bios on start up). When you start the computer and the bios screen pops up(screen with manufacturer logo) start hitting f2(its almost always f2, but may vary based on the manufacturer). This should load it into bios. First things first is on the main screen or first tab, check to make sure there isn't a "secure boot" option. If there is, change it to disabled. This may also be under the bios "system configuration" or "system security" tab, again, all dependent of the manufacturer. Next, head over to the Boot Order or Boot configuration tab and change any option you see set as "UEFI" to either "Legacy", "UEFI or Legacy", or "auto" (preferred). While in there, you may as well go ahead and change the primary boot device to whatever media you're trying to install w7 from, whether it be flash drive or cd. You should now be able to install w7 without a hitch.
Tl/dr, w8 is unnecessarily complicated.
I like it to use, hate it to work on/fix.