It seems that there is no consensus in the proper way to deep clean carpets. What we do know is that a vacuum cleaner won’t do the trick. When you need to properly remove mold, allergens, dust mites, and light stains, you should use a steamer. Steamers don’t require the use of chemicals or cleaning agents although you may be asked to purchase them when renting the steamer. A proper steamer will heat the water to at least 240 degrees and this heat will emulsify grease and chemicals which are then removed by the cleaner. The heat will also kill dust mites and other organic bugs that you don’t want in your carpets.
For deeper stains you should spot stain areas but beware of using chemicals as this can bleach carpet and create a more difficult problem to remove.
How to Deep-Clean Your Carpet [eHow.com]
Steam Cleaner FAQ [Allergy Consumer Review]
Top 6 Carpet Steam Cleaners [About.com]