Hey Martian - great post. Textbook quality!
I would also add: switch cables at the modem/router. Just swap the one that's not working with the one that is and recheck.
Also, see if your network activity lights are working on the machine that has no net connection. Do you have blinkies? If you hvae blinkies, then it's likely software. No blinkies? Likely machine hardware.
And while I would suggest downloading Spybot like simon says (LOL) we are forgetting that the 'net connection isn't working at that machine. Otherwise, I woudl head out to
http://housecall.trendmicro.com/ and try a virus and spyware scan. It's FREE, ladies and gentlemen.