Here is one of my infdev world maps, this one features a large farmhouse for wheat. The farmhouse also includes 3 tunnels one to a water well, another as an escape tunnel, and another to the bottom of the map. I started a nearby forest fire, nothing is in danger. I also mined out somewhere but I cannot remember where, I just know I have some iron. Also I raised the nearby lake by one tile so the re spawn is damp. I didn't spend a lot of time on this, its mainly a proof of concept of a design I made. A recent problem is that pigs and sheep spawn inside sometimes and they ruin some crops. Also, I've made better irrigation.
For Windows Vista locate C:\Users\[USERNAME]\AppData\Roaming\.minecraft\saves
AppData IS A HIDDEN FILE, GO TO PROPERTIES AND CHECK SHOW HIDDEN FILES TO SEE IT
For others I suggest run a search for folder .minecraft or World1
Place the folder inside the .rar in the saves folder;
Right now you can only have 5 worlds, you have to rename the folder from World# to any of...
World1 World2 World3 World4 World5
If you replace an existing folder you will lose that save, make a copy elsewhere if you want to preserve it!