The best thing you can do if a dog is challenging you that way is to let it know who is the dominant one.

Now obviously here, the dog saw you going in circles and came to check you out because it didn't understand what was going on. Were you playing? circling a prey? wounded? You said it was barking, but what was the rest of its body language? Actually, dogs are often most dangerous when they are silent, the calm before the storm, the dog is sizing up the situation.

The best thing is a good solid calm loud, "No!". It's a word that virtually any dog understands. I've found that getting a mouthful of water from my water bottle and spraying the dog with it, if it's at close range is also an effective technique. The dog isn't expecting that.

Another good thing to do is to pick up a "Dog Whisperer" DVD at the library and watch it. Caeser knows his stuff.