I turned on my computer and it says system disk error, replace and hit any key. What does this mean?

What OS are you using, XP or Vista?

-Do you have a floppy disk drive and a floppy disk maybe in the drive, if so remove floppy

-Same thing with CD rom drive, remove CD if one happens to be in the drive

If using XP, a repair installation may be what you need to do if above do not apply

Repair Installation
A Repair Install will replace the system files with the files on the XP CD used for the Repair Install. It will leave your applications and settings intact, but Windows updates will need to be reapplied.

Perform a Repair Installation (http://www.microsoft.com/windowsxp/using/helpandsupport/learnmore/tips/doug92.mspx)

Also check that your boot order has the hard drive listed in the list, you can also try fixing just the MBR of the drive before your try a repair install

To fix the MBR, Boot from your XP CD and choose option "R for recovery console" on the first menu, then follow the 2 questions it will ask you (OS to work on and administrator password (usually blank).

Then when you get to the c:\> prompt type "fixmbr" no quotes and press enter, follow what it says and then reboot without the CD in the drive, if that fails then try doing the same thing and running "fixboot" instead, if that fails also then try the reinstall

Turn your machine back on. If your computer still does not boot up,. a floppy disc has been left in the drive - just remove it and hit any key to restart...