Where does the mobo and CPU get most of it power from? By asking this I mean which rail does the mobo and the CPU draw its power most? 3.3V, 5.0V, or 12V rail?

Well the problem I have is that I think my custom sproggy doesn't have enought power on the 5V (10Amp) rail for the Asus A7N8X-VM/400 with AMD XP mobile 2500. The PSU has 5A on 12V for mobo/CPU/Lilli (HD and DVD ROM on a different 12V rail). The PC run fine with either the engine on or off, but freeze when I crank the car. The issue I know is the HD doesn't get enought power to keep spinning. So 12V isn't an issue because 5A is more than enought for the HD. What left on the HD is 5V with share with the mobo and CPU.