Most likely you're getting condensation lodging around the air cooling matrix or in the plenum chamber. This happens at the end of a journey when you've been using the air-con and then the moisture gets picked-up at the beginning of the next journey. On more expensive cars, like Jaguar and Mercedes, you will find arrangements for letting this condensate drain away - that is why you'll sometimes see a little water drip from beneath them after a journey. On a cheap car which is built down to a price something has to give -and this sort of thing is the result. Interestingly, a favourite problem on Jaguars is that after about ten years or so those condensate drain pipes can get blocked so the air chamber gets a pool of water lodged in it. As soon as the car moves off and around the first bend, this pool of water runs out of the footwell air vent and onto either the driver's or pasenger's foot ! Luckily it only takes about half an hour to unblock those pipes. With the corsa you get a misted windscreen - but if you move up the range to an Omega you have the drain pipes.