If you smell anti-freeze/coolant coming out of your vents inside the car and your windows are fogging up, then yes, it sounds like you have a heater core leak.
Don't freak out though - unlike many other cars, access to the heater core is a breeze...
Search the forums or Google "Boxster heater core replacement" and you'll find procedures on how to replace it. There's also a good write-up with pictures on Pedro's board here:
Re: Heater Core Replacement - Cost? Diagrams? DIY?
Hope this helps!
Bill