I like to think I know more about cleaning carpets than most people, and perhaps more than many professional carpet cleaning contractors. I will give you two pieces of reliable information that flies in the face of what most companies like Hoover, Bissell, Eureka, etc. are suggesting in their advertising (they want people to buy those mini carpet shampoo'ers they make).
#1: The notion that a carpet shampoo'er will clean a carpet better than a vaccuum cleaner is a very common misconception. Carpet shampoo'ers are not and never were intended to do the job of a vaccuum cleaner. A vaccuum cleaner works well at removing solid soils from carpets, like dead skin cells, sand grains, pollen particles, road grit, salt crystals, etc. A carpet shampoo'er will have a very hard time removing solid soils like these because as soon as you get the carpet wet, the surface tension of the water film on the pile will cause these solid particles to want to stick to the pile of the carpet, and that makes them much harder to remove.
Next time you're at the beach, try cleaning sand off of wet feet and dry feet and see which one works better. Hint: The sand literally falls off of dry feet, but it's like fighting with a bear to clean sand off of wet feet. Why? Cuz the surface tension of the water film on your feet tends to cause the wet sand grains to "stick" to your feet, and that makes them much harder to remove, just like solid particles in a wet carpet.
Carpet shampoo'ers are only meant to be used AFTER vaccuuming. The vaccuum cleaner removes the dry solid soils, and the purpose of the carpet shampoo'er is ONLY to remove any dried up liquids spilled onto the carpet, not to remove the solid particles. If you shampoo a carpet without vaccuuming first, you'll remove the dried up liquid stains, but not the solid particles that will remain stuck to the pile inside the carpet. You need a vaccuum cleaner to do that.
#2: Mini-carpet shampoo'ers like those sold by Bissell, Eureka, Hoover and others simply don't have the suction to lift dirty water out of the carpet. Rental carpet shampoo'ers like Rug Doctor and Easy Off have enough, but just barely if you only shampoo the carpet once.
Just in the same way that car engines are classified according to the number of cylinders and the horsepower output of the engine, vaccuum motors are rated according to the number of stages and the "inches of water lift" at sea level. A typical vaccuum cleaner will have a single stage vaccuum motor and under ideal conditions might lift a column of water about 30 inches at sea level. A higher quality wet/dry shop vaccuum cleaner, commercial vaccuum cleaner or rental carpet shampoo'er will have a two stage vaccuum motor that will lift a column of water about 60 inches or so. Typical entry level professional carpet cleaning equipment will have a three stage vaccuum motor or dual two stage vaccuum motors piped in parallel, and either of these will lift a column of water about 90 inches. My carpet extractor has two three stage vaccuum motors piped in parallel, and I've seen portable carpet extractors and flood recovery equipment that have three three stage vaccuum motors in parallel. Truck mounted units are available that use a small gasoline engine to drive 6 to 8 three stage vaccuum motors, and you need such strong vaccuums for truck mounted carpet cleaning equipment because of the long hoses used.
Now, your typical Rug Doctor will have a single two stage vaccuum motor, and your typical mini-carpet shampoo'er from Bissel or Eureka will only have a single stage vaccuum motor. It stands to reason that the more soiled water you lift out of a carpet, the cleaner the carpet will be after it dries. Consequently, the trick to getting the dried up liquid stains out of a carpet is to either have a LOT of suction to remove as much of the soiled water as possible, or to shampoo the carpet where the stain was and lift clean rinse water out of the carpet multiple times, thereby removing more of the dirt each time.
(and now #3, kinda
But, considering that carpet shampoo'ers are only intended to remove dried liquid stains, many homeowners already have the perfect machine to substitute as a carpet shampoo'er already... a wet/dry vaccuum cleaner. Your typical wet/dry ShopVac will have a two stage vaccuum motor and will generate enough suction and air flow through the vaccuum hose to pull soiled water out of a carpet. You just stick the 2 1/2 inch diameter hose directly on the carpet pile and let the vaccuum cleaner draw air through the carpet and the soiled water will be removed as a water mist in the air flow. (It's best to physically "lift" the carpet up with the suction of the hose a bit so that air can flow under the carpet and then through the carpet into the suction hose. If you press down on the suction hose, you won't get air flow THROUGH the carpet, which is what you need to remove the soiled rinse water.)
So, in summary, you can do as good a job cleaning your carpet with a good quality vaccuum cleaner (to remove solid particles) and an ordinary wet/dry vaccuum cleaner (to remove dried up liquid stains) as you can by hiring a carpet cleaning contractor.
And, finally, to put the cherry on the cake, you don't have to know what cleaner to use on what type of stain, and professional carpet cleaning contractors don't know what's in the cleaning chemicals they use. You can do as good a job using a wet/dry vaccuum cleaner to remove stains from carpets as any pro simply by going to any of the janitorial supply places listed in your yellow pages phone directory under "Janitorial Equipment & Supplies" and buying a "spotting kit". A "spotter" is a quantity of cleaner made specifically for removing a certain kind of stain from carpet or upholstery. You can buy spotting kits containing anywhere from 8 to 18 different spotters, and each spotters will be made specifically for a certain kind of stain. You typically get spotters for: blood and other protein stains, vomit and other acidic stains, grass, toner or ink, urine and feces, red wine, the tannin stains like coffee and tea, rust from steel furniture legs, and a general all purpose detergent for use on unknown stains. There is a spotting solution for every different kind of stain, and you don't have to buy a whole kit. You can buy just the spotter you need, which is what the pro's do when they run out of the more commonly used ones, like the spotter for pet urine stains.
Here's a picture of a typical spotting kit that might be sold at a janitorial supply house. You can buy each of the 8 cleaning solutions separately as well.
And, remove your stain using the wet/dry vaccuum cleaner exactly the same way you would using professional equipment; spray the stain with the spotting solution using a spray bottle. Work the cleaning solution into the carpet pile with your fingers. Allow several minutes for it to work on the stain. Then vaccuum out the soiled cleaning solution with the hose of the wet/dry vaccuum cleaner. Repeat as necessary. Spray clean rinse water down, and recover the soiled rinse water with the wet/dry vac hose. Repeat again just to get all the cleaning solution out.
So, if you know what you spilled on your carpet, just go buy a spotting solution for that kind of stain from any janitorial supply store, and you can do just as good a job removing that stain from your carpet using a wet/dry vaccuum cleaner as any pro could with a professional grade carpet extractor.
And, if you don't believe what I'm saying, just phone around to some of your local carpet cleaning contractors and ask them who the Janitorial Supply stores in your area are with the most knowledgeable people on carpet cleaning. Print off this post and show it to the people you're referred to and get their comments.
Hope this helps.
Post again if you want to know the best way to clean a carpet using professional equipment.