Dell Latitude D510
to boot from USB drive
Before powering on, press and hold the f12 key -- do not let go. As you hold the f12 turn the power on. Do not leg go of f12. Eventually you should see a screen with a menu to let you choose the boot device -- hard dish, CD, USB, network, etc., or edit BIOS. Now you may let go of the f12 key. Select the USB boot option.
If this does not work with the f12 key then try to the same thing with the f2 key. If that does not work, then try the same thing in the Del key. If that does not work then try the same thing with the esc key. If that does not work then throw computer in trash and buy new computer. This is a very old computer, which appears to have different BIOS versions for the same model number (therefore, shouldn't they really have different model numbers? This alone would make me want throw the computer in the trash.).
I waas able to get Xubuntu 14.04 installed on a D510 with 1 GB RAM and 30 GB hard disk (on only a 12 GB free partition).