Take photos of every single BIOS screen. Unplug PSU, clear BIOS, fire the machine up and update to latest BIOS. From within the BIOS, and after updating the BIOS, clear all settings again.
With motherboard now at default settings, concentrate on getting the harddrives recognised in the BIOS and see what happens when you load Windows or when you load from a Windows 7 setup disc. If it fails loading the currently installed Windows, perhaps that installation is corrupt. If it fails loading off the setup disc, then it could be hardware.
If you have a multimeter, check the voltages (yellow = 12, red = 5v, orange = 3.3v). Try swapping the following one by one, to determine which might be faulty (assuming you have these spare): harddrive, graphic card, memory, CPU.