How to Boot from a Linux Bootable Flash Drive Using an Acer Aspire XC Desktop Computer
N.B: Acer Aspire XC Desktop Computers appear to have unconventional BIOS access and behavior.
(1) A press of the Delete key at the first appearance of the Acer logo screen will access the BIOS utility, where you should first set a user password. Without a user password, some menu items will be inaccessible.
(2) Once you've got access to all of the menu items, you'll need to ensure that you're set to use the AHCI drivers for your system drive, otherwise you might have trouble installing Linux to the system drive.
(3) In the BIOS utility, you'll want to make sure that you are in setup mode, instead of user mode, and disable Secure Boot ENTIRELY, as Secure Boot is more trouble than it's worth.
(4) Finally, set up the appropriate booting order. This situation may allow the computer to boot from flash drive--but it may not. See (5) below.
(5) If turning off Secure Boot and moving removable media to the beginning of the booting order does not allow you to boot from flash drive, do this: Make sure your flash drive is plugged in, and, at the first appearance of the Acer logo screen, instead of pressing the Delete key, press Fn+F12. This should bring up a DIFFERENT BIOS screen that lists both the system drive and the bootable flash drive, so that you can manually select the bootable flash drive to boot from. Again, the magic keys are Fn+F12. The Fn key is near the Ctrl and Alt keys; the F12 key is near the Prt Scr key.
I am posting this as a thank you to the fine folks who posted their solutions, which steered me in the right direction to solve this problem and literally saved my computer. I was THIS CLOSE to getting rid of the Acer and going with a different brand. Also, it's useful for me to document this recent experience, so that I cannot forget my solutions. . . .