There is little that makes your surroundings more antiquated and unlivable than dry rot. It's not uncommon to find a window frame or sill suffering from dry rot. Ironically this condition is not caused by the wood being dry, but rather, wet, usually for an extended period of time. Here is what probably happened.
The water entered through a joint between the window frame and the exterior shingles. Quite simply, the caulk has failed. This kind of repair is made by first removing all of the rotten wood, then hardening any adjacent surfaces that may have been affected and finally filling the void, in this case, with solid wood and a filler called polyester resin.
It is a job for a true handyman. And, no matter how you slice it, someone is better equipped to do this in your home than you are! That someone in the Dallas area is Home Pro Services serving the Metroplex with a variety of professional, guaranteed handyman services for many years. Just call them at (214) 886-3326.