^That.
To over ride the Windows 8 UEFI, you need to turn the PC off then remove the battery for about a minute. When you turn it back on, you should be able to access the BIOS upon loading the splash screen. The typical key is f8 or f10 on most PCs but I've also seen it be f2.
Once you're in the BIOS, find the boot section and set the DVD drive or USB above the HDD in the list, depending on which install medium you want to use.
Next you should insert the medium and hit escape, then select to exit saving changes. When the PC begins to boot it will do so from the install medium.
At the page where you select your install location, click advanced then select delete. Entirely delete the partition with Windows 8 installed and then create a new one and format it.
Select the newly formatted partition as the install location and click next, it should now be installing Windows 7.