I have had all sorts of problems with windows 7 and the gs3 connecting when using Kies. So much so that I've quit trying to use it and just install the drivers only. Another trick I've learned on the AT&T forum is to block windows from updating the drivers. My phone and computer would connect for a day or so and then would stop. I would have to remove and reinstall the drivers to get it to work again. Then a couple days later it would stop working again.
To keep windows from updating the drivers:
-Right click on my computer and choose properties.
-advanced system settings from left side of window.
-from the hardware tab click "device installation settings"
-select "no,... Never install driver software from windows update.
Remove kies and samsung drivers.
Then install drivers from here.