Sometimes when I attempt to start up my computer, a flashing question mark appears on the screen. What could be causing this?

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