One of the first things that hubby and I do when we're spring cleaning is STEAM & Shampoo the carpets. It's amazing how dirty they can get within 6 months, especially if you have pets like we do.
We don't use any fancy products at all, we literally rent a shampoo carpet cleaner and use a pre-mixed solution with EXTREMELY HOT water. That's it. When it's all said and done, the carpets take a full day to clean, and a full night to dry. Try not to walk on the carpet if you can during the drying process.
We also move all of our furniture out as well, there's no point in cleaning carpets if you can't move at least the lighter pieces.