[tonyvdb]

Are you certain that your out putting a bitstream signal form the HTPC and your using HDMI as the connection?

 

[tonyvdb]

and you are 100% sure that the HTPC audio card supports DTS MA and TruHD audio?

 

[tonyvdb]

I highly doupt its the receiver, is there someone close by that could lend you their BluRay player so you could test the receiver?

 

[tonyvdb]

It may very well be the Handshake between the PC and the Onkyo is not working. Are you sure you are getting HD video out of the PC? It is clearly not the Onkyo that is the prolem or the PS3 wouldnot have worked either.
If your A/V card from the PC is not HDCP compliant this can cause all sorts of issues.