I just built a pc and i am trying to install windows 7 but i am unfamilliar with the gigabyte bios i have, i have tried loads of different things but it always comes up with 'reboot and select proper boot device'
Well if you have a Windows installation on a flash drive, when you're in the POST stage, you can press DEL (I believe) and choose select the USB as the first boot drive (sometimes named something along the lines of HDD-USB)
If you do NOT have Windows on your flashdrive, and want to put it on a flash drive, then go here: http://www.microsoft.com/en-us/software-recovery
Download windows with your Product Key and download RUFUS and use a USB and put the ISO on the flashdrive
First make sure your USB drive has been configured properly to be bootable. Use https://rufus.akeo.ie/ to make the W7 iso bootable. Also ensure your usb drive is formatted to FAT 16 or FAT 32.
Now with your PC turned off, insert your USB stick into a USB 2.0 port. Then turn on your pc and press delete repeatedly to enter BIOS. Next, when you enter the BIOS, go to the boot options (should be under bios features):
Set your Boot option #1 as your USB drive which should be detected (if not detected, turn off pc , use a different port and try again)
If it still can't boot you can do the boot override but I'm sure the above should be enough to get it to boot successfully.