BIOS Issue?

Hello everyone... I sincerely apologize if my question has been answered already in another thread, but as a first timer on this website I'm learning as I go. I'll do the best I can in explaining the issue I'm faced with in hopes that the knowledge of the computer community can help me.

Computer is a Dell Dimension E521 operating Windows Vista. Appears to have the Phoenix Bios System

1. On start-up the computer displays the usual Dell splash screen with the two options of... F2 Setup and F12 Boot Sequence.

2. This is followed by a second screen that displays the following..."Alert! System battery voltage is low". This message is followed on the same screen by, "Press F1 to continue, F2 to enter setup".

3. If I press F2, the computer will go to the BIOS screen while if I press F1 the following message is displayed..."No boot device detected, insert system disk and press F1 to continue or press F2 to setup menu". Unfortunately no system disk is available that I'm aware of. Being a Dell the operating system restore option is present which I believe took the place of any physical disks.

4. If at step 1 I begin pressing F12, it will still take me to step 2, but if at step 2 I press F1 and immediately press F12, it will take me to the Boot Device Menu.

5. At the Boot Device Menu, it displays the following options...
Normal
Bootable Hard Drive
Onboard CD-Rom Device

System Setup
HD Diagnostics
Boot to Utility Partition

6. If I choose Bootable Hard Drive, it then takes me to the Hard Drive Boot Menu. Two options are present...
Sata-0 : Samsung SP2504C
Bootable add-in cards

7. By choosing the Sata-0, it will load Windows Vista as if there were never any problems.

8. I have no idea why the operating system works, but yet the system isn't booting up properly. Any help or feedback would be greatly appreciated.