I'm no expert on the XFire, having just become a new owner.

If the fogging is real severe (in any type of weather), I'm leaning towards Dave's assessment on the Heater Core. If there is a leak in that area, you will definitely get fogged windows (been there on that with another one of my cars).

A typical sign of Core issues is that the car will fog very nicely even after the engine is shut down (and hot) and the car sits with the windows closed...

I had this with my Lincoln Mark V, and the whole dashboard had to be dropped for access, in order to repair/replace.

If it is the Core, you don't want those chemicals floating around your interior, it's not good for your health...