Has it got a restore to factory settings on it.Usually pressing F10 or F11 at start up would take you there.
Have you tried safe mode.tapping F5 or F8 at start up would get you there as well.
Check in the bios that it is starting from the correct drive.it probably does if you can see it on screen.
Was there any problem with it losing time or date before this happened.
And have you tried with the battery out and just using the mains Power cord.