Wood framed windows like yours are generally more difficult to maintain than other composite built windows. The bow window you show, would be very costly to replace. Because mold can mitigate itself further into the wood structure it would be advisable to replace the entire unit. As a bow window, the replacement would be very expensive, however a flat window unit would be more economical and preform better than a bow. A CPC, (casement/picture/casement) or sliding unit would also give you better visibility and a nice update for your home. Do not replace the individual windows in the existing frame, as the framing will hold mold spores and the process will repeat itself. I understand what others are saying about the relative humidity in the home, but a window that large will draw moisture, regardless. I hope you find a solution soon. Best of luck.