The VGA cable has nothing to do with a composite connector!
The VGA cable is simple. The one end goes into the XBOX 360 and has an optical audio jack on it (like the HD Component Cable does). On the other end it has a VGA connector (plugs straight into most LCD panels, may require a female-female gender changer (like $5 online, I think one comes with the official cable) to connect to CRTs with hard-wired cables. It also has "RCA" phono plugs for left and right (the same kind of plugs that the Component/Composite cables have). You may need to get an adapter that allows you to connect these to a 3.5mm minijack connector (headphone plug) that your PC speakers probably have. I'm not sure what kind of extra connectors the official cable comes with (I bought a cheap off-brand imitation one on eBay that works just as well, and I already had a ton of extra adapters and connectors, so I didn't need the ones it came with). You may need to go through different connectors or cables to get the audio to the right kind of connector to match your speakers. RadioShack would be a good place to go if you need to find connectors for audio. If you don't have a RadioShack or live outside the US, you can look for these online.