Re-flashing a BIOS is not a normal thing to do and if the BIOS is really
corrupted you probably can't do it anyway as the BIOS is necessary to start
the computer. Why do you think you have a BIOS virus; is there some
behaviour that leads you to suspect that, or are you just naturally
paranoid? ;-)

You have root control in effect every time you install. There are always at
least 2 options - take over whole drive or install side by side with what's
there already. A manual install isn't that difficult but you do need a basic
knowledge of he Linux file structure.

There is no "root" user by default in Ubuntu. It uses sudo before the
command to do root things, including sudo bash. It prompts for a password,
which is always yours, the one you logged in with. Exit or Control-D takes
you back to regular user status.