I replaced the leaking valve leading to the hot water heater, drained the hot water heater, turned the main valve to the house back on and now have very low water pressure to the house (cold and hot) The house has galvanized metal pipes... HELP please!:confused:

If EVERY valve in the home has low pressure I would suspect the main valve you turned off. Opening and closing this valve may have freed up some corrosion which jammed inside the primary gate valve or a pressure reducing valve. If it's just certain valves (not the outside hose bibs for example) you should check the aerators inside the spigots at the sinks etc.. You will have freed up corrosion and the screened portion at the discharge will catch all the rust and debris. If the plastic tube inside the tank disintegrated those plastic pieces will flow to and restrict at the same screens.
Good luck
AHIINC