I have a 2007 Rav4 and now that the temperature is below zero the inside of the front windshield frozes up. How can I fix the problem?

Warm up your car and defrost the windshield. Make sure you defroster is on full.

Keep the inside glass very clean. I use "Invisible Glass" cleaner.
Keep as much moisture off the carpets as you can.
Do NOT use "recirculate". Use 'fresh air' mode. (defrost will default to this) It's the best to use with 'heat' all winter.
The less moisture inside the cabin the better and will drastically cut down inside frost. Don't forget about that cabin air filter too. Be sure it's replaced often.