What is the HDMI from your PC connected to? direct to a TV or to an AVR?
The Audio Output at the has nothing to do with how audio is actually routed out of your system, it merely specifies what options are available in the settings GUI.
When Audio Ouput is set to Analog you're telling XBMC your audio devices do not support passthrough of formats such as Dolby Digital & DTS, thus it greys out all the "capable receiver" options and the "Passthorugh output device" option, this forces XBMC to decode all formats in software to LPCM.
When set to Analog the "Audio output device" still tells XBMC where is audio is actually to be routed, so can still be any of your available audio devices including HDMI.
When Audio Output is set to HDMI then all the "capable receiver" options and the "Passthrough output device" become available, as default most of the "capable receiver" options are enabled for the case where you are connecting to a AVR. If you experience problems with HDMI selected then it's likely you're trying to passthrough a format that the device connected to the HDMI, either AVR or TV, does not support, or the "Speaker configuration" is incorrect for the number of available channels for the HDMI link.
Refer to the sticky to do more investigation, see
http://forum.xbmc.org/showthread.php?tid=146911
!st thing to check is the Supported Formats tab for your HDMI audio properties within Windows, you need to see the number of channels reported and what encoded formats are shown.