Mac Pro 1,1 (2006) boots straight to desktop, therefore no boot loader or Apple Logo

Hello,

I have a Mac Pro 1,1 (the first ever Intel version) from 2006 who has Mac OS X Lion installed, and it has a strange problem:

when I boot it up, I hear the usual startup chime, and then NOTHING (screen in energy saving mode) until the login screen/desktop, so no Apple Logo with the loading wheel.

That remains a merely cosmetic problem until I have to access the boot loader function to choose from which disk to boot: even if I press the "Alt/Option" key, it does not show up, but somewhat "hangs" the boot process, so Lion does not boot; i assume therefore that the boot loader "loads", but it can't show up on the screen (in fact all the boot commands like C or PRAM reset - already tried without success - who does not have a visual input work flawlessly) for some reason.

Even if I try to boot from a Snow Leopard Install DVD (Retail) with the "C" key, I hear the DVD spinning and loading the installation software, and even if I click blindly I hear the DVD spinning again (loading what I clicked I assume), so this is definitely a video showing problem.

The Video Card on this system is an nVidia GeForce GT 120, and as far as I know, no change has been made to the original boot loader.

So what it could be?

Thank you,

Lupo.