I currently have Ubuntu installed on my computer but want to revert to Windows XP (won't run both because my hard drive is only 20GB, so that doesn't leave me with much partitioning room). I tried getting into BIOS at computer boot up so I could choose the boot from CD option but the computer is not letting me. The disk won't boot up on its own and I can't get it to boot from Ubuntu or don't know how. How can I get XP back on my computer?