When I don't have my car engine running my system works perfect. When I do have my car engine running the pc wont boot, I have gotten it to boot once but it only lasted for a second or three, then it shut down.

I have a invertor which is hooked up to my power cap which is hooked up to my car battery. The only thing running from my invertor is the PC. On my PC I have a 5 port USB2.0 PCI card, a Creative Labs external sound card (USB2.0) and a 3.5 Seagate HD. My TFT screen runs of the power cap (car battery). The voltage on the power cap reads 11.5V when the car is not running AND when it is. The voltage on the invertor reads 180V when the car is not running AND when it is, which should be 220-230V though so it supprises me that the computer even boots at all. But why does it boot when the car is not running and it doesnt when it isnt? Voltage-wise nothing really changes.
I am considering buying an M1-ATX, but would that work on my Compaq Dekspro? I am planning on running an extra slimline DVD player on the whole system and GPS.