Go into your BIOS and make sure that the hard drive with the Windows installed is correctly set as the primary boot device. Choose the HDD as primary boot device, followed by CD-ROM/Floppy. Also make sure that the HDD is set to auto-detect within the BIOS utility.
If you have any discs or diskettes in the CD-ROM or floppy drive(s) then please remove these and try and boot up again, this is to make sure the system is not trying to boot from the said device.
Make sure ALL cables are 100% connected to the system and are not loose at any scale. If any of the above does not work then try and reset the CMOS using the CMOS jumper.
If still nothing above fixes the issue then you may have to replace/fix the master boot record on the hard drive as this may be the cause. Post back after you have tried the suggestions above.