How can I re-socialize my dog so she's not aggressive?

We have a 2-year-old Queensland Heeler that we got when she was just a puppy from the Humane Society. We took her in and she graduated from training class and she was very well behaved with all the dogs in the class as well as very loving to new people. Just recently, whenever someone comes through our door, she will bark, growl, and snarl at them. My sister just got a new dog and I took my dog over to socialize with her but they ended up in a scuff. We separated it before it got worse but now my dog won't go near other dogs except our little Italian Greyhound that we own. I can't take her to the park or anything anymore because I'm afraid someone will want to come up to her, or someone will let their dog off their leash and come running up to her. What can I do to get her socialized again where she knows to behave with other dogs and people?