There may be a motherboard failure of some sort, but you can try a fundamental test since you are happy to open the case.
Remove the power supply from the HD, the Floppy drive, and any CD device you have.
Make sure the video card (if it is separate) is properly home and secure.
Now switch on. (check the CPU fan and PSU fans are running.
If you are lucky, the monitor should show something leading up to the point where it cannot load Windows "for a variety of no disk type reasons."
If you get this far, the fault lies with one of the disconnected devices, and you can try re-connecting them one at a time starting with the HD.