Dell Demension 4600 - I need help

Hello, I have a Dell Demension 4600, the hard drive dies. I put in a new Hard drive and sometimes at power up I get either a blank screen with a flashing curser in upper right corner, or the F2 F12 to hit and when pressing these I can't always get tothe bio screen. If I get to the bio screen, I can hit escape and get to windows and work fine, but how do I solve this problem so my system works correctly?