To keep water from getting inside your window trim and causing wood rot to the trim and sill, install a window drip cap. It is installed at the top of the window, and divers water away from the window during a rainstorm. You can buy preformed drip caps, and with little more than a hammer and nails, you can install them fairly easily.
Measure the width of the window. Purchase a preformed plastic or aluminum window drip cap from any home improvement center or window retail outlet. If wood rot is present, replace the wood there.
Pry up or remove a row of siding from above window. Remove any debris. Cut drip cap to a length slightly wider than the window. Apply flashing tape above window. Slip cap under siding against flashing tape.
Nail the cap to sheathing with galvanizied nails, making sure the cap slopes away from window. Caulk along top edge of drip cap. Cover nailheads with housewrap or felt cover, or use roofing tar on each nailhead.
Cut both edges of drip cap overhanging width of window lengthwise with tin snips to make two flaps. Fold in so wind can't blow rain under the cap.
Apply exterior-grade sealant along edge of drip cap against the house. Re-attach siding. Leave a 1/4-inch gap over the window so water cannot seep under the siding. Caulk both ends of the drip cap.
You can make your own window drip cap by bending it into shape from sheet aluminum. Clamp sheet between two blocks of wood in a vice, and tap with a mallet to bend.