These images show the process of repairing a rotted corner in an otherwise sound hardwood window frame. This rot occurred almost certainly due to the failure of the rubber draught seal that is fitted to this type of window. Over time, the rubbers can become compressed and tired, which weakens the seal, allowing water to ingress and resulting in the rot. You can replace this seal relatively cheaply. It can be purchased in rolls of 50m and you simply cut it to length, then fit it after removing the old seal. Be sure though to get the correct profile (the shape of the cross section). Most suppliers will send a sample on request so that you can make sure. Catch this problem before it starts and you can save a lot of money and effort in having to replace or repair window frames and cills.