Change the main computer? that's crazy-talk!
The clutch pedal switch is often the culprit. If you get on your hands and look closely under the clutch, you'll see a slider on the rod underneath the pedal. It is green on a gen2. not sure on a gen3. anyway, if that sensor doesn't show that the clutch is all the way down, you can't start the car. many people just ground this out so that the computer thinks the clutch is always pressed in... but then if you press START with the car in gear and there happens to be a wall in front of you... OOPS. So don't do that. But check out the switch and maybe replace it.
good luck!