Mold on windows is usually caused by cold outdoor air on the other side of the window cooling on the glass causing condensation. Although mold cannot feed on the glass itself, if the window is dirty then mold can feed off of the debris on the window glass and frame material.

Condensation can also lead to mold growing around window frames because of moisture condensating on the glass and then running onto the frame. The other place to watch out for mold growing around your windows is in the seal where the window frame meets the wall. And of course older windws may be allowing water intrusion. This is usually found at the lower corners of the window.