It is likely that you are holding down the button on the mouse while starting up the computer. Holding down the mouse button at startup causes the computer to attempt to eject a floppy disk. However, if you do not have a floppy disk installed, the flashing question mark appears. If you release the mouse button, the startup process will continue normally.
For more information on troubleshooting a flashing question mark, please see:
Article 13784:
Flashing Question Mark on the Screen: Causes and Cures