Why does my screen flash a file with a question mark on a white screen

That symbol means the computer can't find a system folder to start up from. Sounds like either there's a hard drive problem or it's missing its hard drive. Is this a computer that was working properly earlier or is this a newly acquired computer that's never worked properly?

You could try restarting with the option key pressed down -- that should force the computer to show the available system folders from which it can start up. But I'm guessing that it won't show anything, since if it could find a working system, it would start up from it...

I'd recommend starting up from the original OS X install disc that came with the computer. Insert the disc into the computer and press "C" while starting up. If that works, you may be able to see your hard drive. If you get to that stage, you should immediately back up any data you might want to save from that drive to an external hard drive.

Then you might be able to use the utilities on the OS X install disc to check out the hard drive. I believe that install disk has Disk Utility, which would be a good tool to start with for disk repair.

While it's likely that your hard drive has failed, it could also be the system is entirely broken. One way to test this is to boot up to the system DVD, and open Disk Utility (it's in one of the menus). Does your drive show up? If not, it's probably failed. If so, immediate use the Restore function to clone your internal drive to an external drive. Then proceed to attempt repairs on your internal drive.