Booting comes after POSTing. When you boot, you should see a line analogous to this:
. You should be able to hit the e
key to edit the corresponding entry. The line identifying the kernel starts with the word linux
; on my system, it looks like this (and spans multiple lines on the screen):
linux /boot/vmlinuz-3.2.0-0.rc1.git4.1.fc17.x86_64 root=UUID=74c220e6-a864-45cc-8a18-0837303c3d1a ro rd.md=0 rd.lvm=0 rd.dm=0 KEYTABLE=us SYSFONT=latarcyrheb-sun16 rd.luks=0 LANG=en_US.UTF-8
Somewhere within that line (or at the end), type nomodeset
(be sure it's surrounded by spaces), then press the F10 key to continue booting.