Q: where are the rented films once downloaded??   i can't find one thats just downloaded!!

If you rented them on an iPad, iPhone or iPod Touch then they should have downloaded into the Videos app. If you rented them on your computer's iTunes and they don't show in the Movies part of your library then try closing and re-opening iTunes and see if they then appear in it (or if you get a Rentals section).

I've also just downloaded a rental onto my laptop and there's no sign in iTunes ...