I am installing Debian Jessie from netinst on a USB key. I went to BIOS
and made the key first boot priority, but made a BIG mistake and failed
to disable UEFI Secure boot (UEFI is new to me).

As the result, when I now power the machine on, I can only do one thing,
which is to register with Microsoft. I found out too late that BIOS is
now disabled. How do I get access to BIOS back and how do I respond if
and when I'm asked for admin pw?