Strip out all parts leaving only the CPU, motherboard, and necessary power connectors. The system should complain via BIOS beep codes that your system does not have any ram installed. If it does, install one stick of ram. If you do not have onboard video, the system should then complain (again, via BIOS beep codes) that there is no video card installed. If it does, plug the video card back in and see if you get anything on your monitor. If yes, plug in one more ram stick. and repeat until all of your ram is reinstalled. If it still works, plug in your hard drive, dvd drive, and any remaining expansion cards (remember to only do one at a time, then power the machine off before reinstalling something else). If at any point, the system once again fails to boot with those same symptoms you described earlier, you have found your defective part.
If the above recommendation doesn't work;
You have a corrupt MBR / corrupt system files.
You'll get a new master boot record by restoring the system by booting from the recovery partition. Boot from it, run the recovery, and it will format the drive and you'll be back to where you were when the computer was born.
If you haven't backed up your files yet, you can do a full backup even though windows wont load,
just boot from the active boot disk program cd and backup to external drive.