When your PC starts, there's this beep and some black MS-DOS-like screen pops up before windows loads right?
During that screen you'd need to press 'F1', 'F10', 'F12' or 'DEL' to access your BIOS. It usually is one of those anyways.
If it isn't it's written on that screen either way, usually 'press *key* to enter setup/bios/cmos'.
As to disable the IGP, I can't really help you there so you'd have to look around in the BIOS. Search for something like 'PCI' 'IGP' 'Integrated GPU' 'Integrated graphics controller'.
If you can't find it, exit the BIOS WITHOUT saving.
The BIOS is essential to your boot-up procedure and quite a few settings of your hardware. Messing around with these without knowing what they do could mean your PC won't be able to boot anymore or it would be damaged.
