There is mold because there is moisture
Stop the cause of the moisture
Kill the mold (50/50 water/bleach or X-14)
Seal what's left (Zinnser's BIN shellac-based sealer)
Paint with premium paint (I'd suggest a nice enamel, oil-based or waterborne as your preference)
Is this in a bathroom? If it is in a regular room there must be moisture building up on the window and dripping down. It is possible you need new windows if the seal has worn away in the panes. You can tell if the window looks foggy.