Are both radiator hoses hot to the touch? If not that's sure sign of clogged heater core.
Flushing like Mr Crusha said is a good idea. While you're at it, change the t-stat since you'll have the coolant out of the system anyway and they're cheap.
"A veteran - whether active duty, retired, national guard, or reserve - is someone who, at one point in his or her life, wrote a blank check made payable to The 'United States of America', for an amount of 'up to and including my life.'"