Okay; for your problem, what you should do is run the CTRL+ALT+D or hit F12 right when you turn the computer on and see the Dell logo. If this screen will not come up, turn the system off, and wait 30 seconds. If you press the key too early, it will come up with a keyboard failure. For this, simply turn the computer off, wait 30 seconds and try again. Make sure there are no floppy disks in your drive. If you hit CTRL+ALT+D, you should see a message that says "IDE Drive diagnostics running. Please Wait." This should run for a few minutes. If you hit the F12 key, simply choose IDE drive diagnostics or Hard drive diagnostics from the menu and hit enter. Again, this will take a few minutes.
Before you hit enter to continue, take note of whether drive 0 passes or fails. If you come up with a failure (probably return code: 7) your hard drive needs replacing. For this, contact Dell Technical Support if you are still under warranty. They will replace it for you.
The reason I say this, is that a good 9 times out of 10, a missing file like ntldr will turn out to be the result of a failed hard drive... (unless you have a floppy disk in the drive.)