2000 Ford Taurus Coolant coming out of the Overflow

I had a fellow co-worker put collant in my car and he over filled it. A couple of days later it started coming out of the overflow tube. I understand that it should have not been filled past the mark on the tank. However, after it got below that mark it didn't stop coming out of the overflow tube. I have made sure not to let it get to low. It is still doing it. What can my probelm be. I have bought a new cap. so I need to have the system flushed and checked for holes in my hoses, etc? Do I need to change the Thermostat? I really don't know what the problem is because the car isn't running hot. Help!