503 is the error that asp.net gives when apache is up but mono isn't working.
Maybe Wolstech, as the resident windows expert, has some ideas. This whole thing is kind of making me chuckle though because asp.net is a windows thing, and mono was made to run this windows only thing on Linux, and now you're running this thing on Windows that was designed to run Windows technology on Linux. It reminds me of plugging an AC to DC adapter into the wall, and then hooking a DC to AC power inverter to it.