You're going to want to get into the bios of your computer. To do that, restart your computer and press either the delete key or F2 a couple times at the first screen you see when you reboot (should say Acer or Aspire or something, etc.). Then you're going to go under each of the first few tabs and in one of those you will find the boot options. Then just select which drive you want to boot from whether it be the hd, the disc drive, or an external drive.