Although it depends on your BIOS, you can usually access this option through a "Boot" (or something similar) and look for a boot order option. Sometimes this will be on the main screen, just look for a list that looks like this:
1) Removable Device
3) Hard Disk
4) Network Boot
Hunt around long enough and you should be able to find it. Once you do all you usually have to do is move the item up or down the list, the BIOS should tell you which keys to press to do so, as it can very from version to version.