I wouldn't recommend using a PC as your calibration source - there are too many variables involved to get a reliable reference output. Things like video card version, driver version, OS version, playback software, enhancements all make it difficult to know what you are outputting.
Get a DVD/Blu-Ray and turn off any enhancements it may offer - chances are that this will be much closer to reference output than the PC
1. Connect directly to PJ and adjust PJ settings until you get correct results for brightness & contrast - Make sure you turn off any PJ enhancements
2. Now connect player through receiver and verify nothing has changed - if it has, then go through the settings in the receiver, turning off enhancements and experimenting until correct picture is restored
3. Begin your calibration - good luck I am sure you will spend many hours learning, and hopefully you can get a good result.
4. Once happy with calibration, verify that your XBMC looks correct - not sure if you can use the same test patterns as you would with a PC
Alternatively purchase a Lumagen mini - this makes the process much easier and would probably deliver better results.
Have a look at the Display calibration thread at AVS - very good information here.
Hope that helps,