There are three main reasons that this message will occur:
1) You have a floppy disk in your drive - this can be solved by hitting a key which skips the machine from trying to load the floppy disk or remove the disk.
2) Windows has corrupted - This means that you will have to format the drive and reinstall Windows. If you have a recovery disk, this should format the machine for you and reinstall but you will lose everything on the machine.
3) The hard drive is corrupt - Sometimes running scandisk off of a boot disk will fix bad sectors and get your machine running, if that fails a format might fix it. If not then your hard drive is off to silicon heaven and you will have to buy a new one.
It is one of the error messages that you really do not want to see unless there is a floppy in the drive.