I recently salvaged an HP Proliant Server from the trash (I know!!). It was in a fully functioning order, sporting 1 GB of RAM and a 2.6 GHz Pentium IV processor. It also had an Adaptec RAID controller for the hot swap module.
The problem I faced with the machine was when it boots up, the Adaptec controller displays its own boot-up screen - hence closing the default screen. I have never worked on an HP machine before so I had no idea how to get into the BIOS setup to configure the boot options.
I managed to stall the computer with a bootable USB drive and found what the keys were.
So, if you are stuck in the same dilemma I faced, these keys could come in handy.
F8 - Boot Menu
F10 - Setup Menu
F12 - Network Boot
ESC - Summary and Boot Progress
I am happy to say that this machine is now a dedicated NAS for my home network. :D (I also tried pfSense on it but NAS seem to be a more valuable duty for it - too powerful for routing.)