Sometimes the bios will flag this up if you have updated your registry, so the best bet is to go into the bios "F2" in your case, and then look under "boot" and make sure that your drive (or what ever drive you want to boot from first) is listed as the first boot device. When done Select "save and exit" and then when you see a black screen with your bios Name (or computer name) press the button on your computer (usually the big button) and hold it in until your computer completely shuts down. Then wait 20 seconds, then re-start.
Please post back if you have anymore problems or questions.