I always install outlets at 16 inches to top of the box, and switches at 4 feet. The fact that it's under a window should make no difference, but I would have this circuit dedicated with it's own breaker AND have it protected from moisture by utilizing a GFCI. This can be accomplished either by using a breaker equipped with GFCI protection or by having the first wired outlet be a GFCI receptacle, with all other receptacles downstream from this one. This will protect the entire circuit, should water get in during a storm when the windows are left open. As far as placing, the code states every 10 feet, but I don't have the NEC here to quote from. The grounds should all be twisted together and clamped with a crimp on clamp. Make sure the wiring, receptacles, switches, and breaker are all compatible. Typically, 15 amp circuits will utilize 14-3 wire and have plugs, switches, GFCI, and breaker of the same rating. A 20 amp circuit will require 12-3 wire AND receptacles, switches, and GFCI that match the amperage. Unless you're running equipment on the porch, I doubt you'll need a 20 amp circuit. Make sure holes for running wires are through the middle of the studs or use metal conduit to protect the wire if it is surface mounted or visible. Good Luck.