If you cannot do that, may be some problem with boot sector of HDD, boot from CD and go to recovery consol and fix up there, type help will show you option, maybe fixbot and fixmbr.
Still no work, reformat and reinstall is only option.
Still no work, maybe hardware faults, test hardware, one CD called Ultimate Boot CD click here which has a lot of tools for dignostics. You can download, burn it to a CD and boot up from that CD and run some diagnostic tool from there.
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