Hello,you say you just built the system?
You installed the operating system then you put in Motherboard disk that came supplied with your board?
You cannot enter BIOS untill this disk is put in.
Are you using a USB keyboard and mouse?
If you are these need to be enabled in BIOS.If you are using USB keyboard/mouse plug in a normal keyboard and mouse to enter BIOS.
Computer Post / Beep codes