Normally I'd suggest checking the starter if you know the battery/alternator is fine. However, if the car stopped running while on your way somewhere then you can rule out the starter.
I'd double check the battery. Use a cheap-o voltmeter just to make sure it's carrying a charge.
If it is holding a charge, then most likely the connections to the battery have corroded away and need to be replaced. If it's not, then re-charge or replace the battery and check the alternator.
Keep in mind that just because the windows and radio work doesn't mean that the battery is fine. They require so little electricity compared to the starter that they might operate just fine on a weak battery, while the starter won't be able to turn the engine over.
I'd double check the battery. Use a cheap-o voltmeter just to make sure it's carrying a charge.
If it is holding a charge, then most likely the connections to the battery have corroded away and need to be replaced. If it's not, then re-charge or replace the battery and check the alternator.
Keep in mind that just because the windows and radio work doesn't mean that the battery is fine. They require so little electricity compared to the starter that they might operate just fine on a weak battery, while the starter won't be able to turn the engine over.