I have a Dell Optiplex GX150 with Windows XP (I believe it's SP3). Yesterday the computer was working fine. Last night I moved it about three feet from where it was and now when I try to start it up I get the error message that there is no boot device enabled. I have opened it up and reseated the memory boards and cleaned it with compressed air. Same problem trying to start it up. How do I enable a boot device?