I posted this in your other thread before noticing this one. I'm not sure which one will be deleted.
"You should be able to enable whatever is disabled but are you sure you're interpreting it right because obviously your system is booting up? Enabled boot devices on some of the bios' will have a checkmark in the parentheses to show it's enabled. You might want to check your manual.
The PX is probably a network boot option."