Hi,
The phase of the startup where you hit the Del key to enter setup stands completely loose from the OS as the OS is yet to be loaded, besides, even that only happens AFTER the Grub comes in the picture...
I'd suggest to hook up an external keyboard and try with that...
No matter what you do (besides actually putting the laptop on fire) to the system, the Del to enter setup is always available..
Melissa
Edit - I assumed you have a laptop judging from the ASUS...