I've looked into getting this breed myself - an Ovcharka.
http://en.wikipedia.org/wiki/Caucasian_Ovcharka
Ov's were originally bred in Russia to hunt bears and wolves, and protect land and livestock from both predators and thieves. The wiki page has only a shred of info on them, there are many sites for owners and fans, and from what I've read, they're incredible guard dogs. They learn quickly what is 'acceptable' on the property in terms of animals, and will literally remove anything that isn't 'family' to them. They are quiet, and prefer to sit and watch their territory rather than play. Unlike most dogs that bark to repel unwanted guests, Ov's will confront them physically (if your foxes see a 200lb bear-hunter coming towards them, I don't think they'll stick around!) Many owners say that their dogs are nurturing and very tolerant of small children, tugging on tails and ears, etc. Ov's were used by the East German border patrol after WWII, to deter people from escaping to West Germany; their handlers had no need to carry firearms. After the Wall fell, thousands of these dogs were let loose, but instead of becoming vicious pack animals, many of them were adopted by families and adapted well to the family lifestyle. Here's a vid of an Ov playfighting with a dog a fraction of his size:
http://www.youtube.com/watch?v=dL9J8_NV530
Another great guard dog, if you have lots of running space, is the Czechoslovakian Wolfdog.
http://en.wikipedia.org/wiki/Czechoslovakian_Wolfdog It was created in the 50's by breeding Czech German Shepherds with European wolves, to create a very trainable and loyal dog, with the build and stamina of a wolf. Again, they don't like to play much. They are happiest when they are 'working', or accomplishing a task. They won't want to be your lap dog, following you everywhere; they'd prefer to walk the borders of 'their land' and keep an eye on the flock. Here's one enjoying her first snow
http://www.youtube.com/watch?v=K_Sygqj_TVA
And finally, my personal fave and the old standby - get yourself a German Sheperd or a GH cross. They have the weight and size to scare off a fox, they're fantastic around kids, very obedient, can understand a vast vocabulary, playful, and will protect its 'family'. You could even get a pair, as they are pack animals and can coordinate protection of a flock between two dogs. Here's a pretty ballsy chicken attacking a GH, who doesn't really know WHAT is going on lol!