You need to access your BIOS to change the boot order of your computer.
First go here http://www.inventio.co.uk/getbios.htm to find out how to
access your bios. Restart your computer and repeatedly press the key that is
listed in the above website. Once the bios appears, look for a tab that
lists the order your computer boots in (I'll bet your external hard drive is
listed as first). Change the boot order so your computer looks for your
internal hard drive first (the hard drive is probably listed as IDE or SATA).