Right, here's what I would do;

1/ Use ONE OF THESE CABLES to connect your PC's DVI port (white one next to the blue VGA on the back) to the HDMI input on the back, marked "1(DVI)"

2/ Use ONE OF THESE CABLES to connect from the DIGITAL OUT on the back of your PC to a digital input on your Onkyo

3/ Use VLC (or Media Player Classic in conjunction with CCCP, or Zoomplayer - which lately I prefer to both - no idea if it responds to the remote control like MCE does) to play movies, and PM me if you don't get any 5.1 audio from it as I may be able to help you change the settings so you do