First, you will need an .ISO image of a Linux OS that you will put onto a USB drive (4GB+ recommended).
As far as .ISO images go, you can use one from any OS that you wish. We strongly suggest downloading the 'Desktop' or 'GUI' version, however. .ISO images can be downloaded directly or via BitTorrent from one of the major distribution's websites, for example:
Once you've created your LiveCD bootable USB drive, insert it into the computer, reboot, and then enter the BIOS boot order menu to ensure that the system will try to boot from the USB drive before anything else.
Usually you will need to press <del> as the system is booting in order to 'enter setup' or access this BIOS menu. Note that for some systems you will need to hit <esc>, <F10>, or <F11> instead.
Next, use the arrow keys and select Boot to access the Boot Order menu, usually named "Boot Device Select". Here, you will set the computer to boot from the USB flash drive first. Most likely you will see the drive flash drive listed by its manufacturer, or something along the lines of 'USB Mass Storage Device'.
When you have set the Boot Order correctly, press <F10> to Save and Exit and the system will reboot.
If you have a graphics card on the system, you should see an Ubuntu/Other-OS logo and menu appear (GUI Interface). Boot into the system to troubleshoot. Do not install as this will wipe out your entire system.
If you don't have a graphics card or on-board graphics on your system, you should be presented with a terminal login screen. If not, or if you see only a blinking cursor, try pressing <Ctrl>+<Alt>+<F2> or <Ctrl>+<Alt>+<F3> to switch to another terminal.