HELP!! The boxster is overheating.

[JP-s-in st. louis]

Here is the story, just went out for a drive and the Boxster overheated. I put 2/3 of a gallon of distilled water in while setting on the side of the road so I could get home. (here is the strangest part) On the way home I cranked the heat and found that it would blow hot air then cold air without me touching the temperature. I am a tad befudeled by the recent turn of events, originally I thought it would be the thrmostat now maybe the system needs belching or even the overflow tank may need replaced. :ah: Please anyhelp would be great!!