Like bama said.
Add a half cup or so of chlorox or any chlorine bleach. I'd avoid the newer "low odor"
lemony flavored kinds. Fill the water tank. Run your pump and open each faucet until
you get chlorine water out of it. Let it sit in the system for 30 minutes or so.
Drain the system and refill and flush it once or twice.
Don't forget to drain and flush the water heater. They never drain completely and there
is probably a half gallon of water in there that's been there all winter.