At about 17:20 EST, an Aerospatiale 355 medical helicopter was destroyed when it struck a house while attempting a precautionary landing due to weather. The pilot, flight doctor and flight nurse sustained serious injuries. The helicopter departed St. Vincent Hospital to transport a patient from William County Hospital to the Medical College of Ohio. When he was approximately 4 miles to the west of St. Vincent, the pilot elected to return because of weather. While en route to St. Vincent, the pilot received a report from a company pilot who was operating between St. Vincent and William County. That pilot reported 2 to 3 miles of visibility and one or two snow squalls that could easily be circumnavigated. With this information, the pilot turned the helicopter back towards William County. After establishing the helicopter on course, the pilot felt the weather was still inadequate, so he initiated another 180 degree turn to abort the flight. During the turn the helicopter entered IMC. The pilot initiated an IMC descent to regain ground contact. At approximately 60 feet agl the bottom of the helicopter struck a tree. After the impact, the helicopter continued on its descent trajectory and struck a house approximately 30 feet from the base of the tree.