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#21
I'm unconvinced that the A8N-SLI cannot be booted from a USB stick. (I should remember, because I used to own one. However, I handed the machine down, and it has since died.) A little Web searching finds posts from people who claim to have done it.
With the PC powered down, and the USB drive plugged in, press the F8 hotkey. That will bring up a boot menu that allows you to choose a boot device. (It's a one time choice. I've preferred that to changing the BIOS settings.) I'm not sure how the USB drive would appear; perhaps as "removable".
Just for luck, make sure that the USB drive is formatted as FAT32. FAT would be required for a UEFI installation. It may not be required for an old BIOS, but it should do no harm.