Indeed you have. Sorry I missed that.
So the driver and USB hardware seems ok; if I'm not mistaken, that turns suspicion on the microcontroller, bootloader, and avrdude.
Easiest way to check avrdude would be to test your computer with another Arduino Duemilanove. If you're in a big city there is a good chance someone around you has an Arduino; you might also try a local hacker space, robot club, or dorkbot chapter.
The microcontroller and bootloader is a little tricker. If you have another Arduino, you might try using the good one to re-burn the bootloader on the troublesome one.
I don't use Windows so I'm making educated guesses here.
Finally, have you asked DX.com? Do they have a forum or mailing list?