I was having similar issues with a new Windows 7 box hardwired blowing a Win XP wireless laptop off when the Windows 7 machine was booted. The cure was to disable IPv6 on the Win 7 machine. That seemed to cure it. It also means that you can't use a Homegroup on the Win 7 after you do. It runs on IPv6 and the main issue is that most routers won't process it, unless they are new and can handle it.
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