Went inside, and went back out about 7pm and went to start it...it won't start...

When you crank it to the 'Start' position on the key, it turns over and starts fine, but as soon as you let the key go and it flicks back to the 'On' position it stalls instantly...if you hold the key on 'Start' so the starter motor is continuously cranking over it stays running, but as soon as you let it go it dies again...
It is dark, so I haven't had a chance to check anything out apart from the fuses, so I was wondering if anyone knows what will be causing this, so it will make it nice and quick for me to fix tommorrow morning...
Thanks heaps.
