I've had Windows on this Optiplex 990 for a long time, but it was installed MBR-formatted.  Not a problem until I wanted to use bigger disks.  I was able to change to UEFI, boot a windows installer, and get Windows running on a GPT-formatted disk.

Until I rebooted.

Now the BIOS cannot see a bootable disk (other than the thumb drive I used to install it).  I do not know what to put in the Boot Options in the BIOS.

How do I get this thing going again?  I'm also going to want to install Xubuntu on a second drive, and may need instructions for that too.