|Did replace your hard disk or install a second hard disk? |
Did you previously have Windows installed? If so, what version of Windows.
Post the full model of the desktop.
If you didn't mess with any cables inside the case, you shouldn't need to do anything. That said, depending on the model you may need to make some changes in the BIOS settings. We need to know the model.
How are you getting into the BIOS screens?