Where there's both warmth and moisture combined, mould will grow. Mildew is the black and white bi-product of mould that forms a thin layer over whatever it happens to be resting on. Mold and mildew can cling to almost anything as long as the climate is right. Mildew and mould can also lead to respiratory problems if not taken care of.
Lay down a drop cloth near the window to catch any mildew residue. This will protect your carpet from any bleach that may discolour it.
Mix equal parts of bleach and water in a spray bottle.
Spray enough of the solution on the mildew to saturate it.
Allow the solution to sit on the mildew for 5 to 10 minutes.
Use a stiff bristle brush to loosen any mildew that is still adhering to the window frame.
Use a clean damp cloth to rinse the area. Repeat if necessary until all of the mildew is gone.
Vinegar can be substituted for bleach.
Wear gloves and a mask when removing mildew. Ventilate the area well. Dispose of mildew debris immediately. Do not leave it in the house.