According to this story on The Inquirer, Xbox Live is being opened up to be reachable by PCs.  Halo 2 will be the initial test of this, and if it works you will have a chance to pit yourself against console only gamers.
“MICROSOFT HAS ANNOUNCED that the Xbox Live network – originally for Xbox alone – will now be extended to the Windows platform and arrive with the Vista version of Xbox classic Halo 2.

PC games as you may or may not know are now branded Games for Windows over in the States – now, Windows gamers will be able to pick e-fights with console players and ‘puter users alike. Microsoft has had significant success with its Xbox Live, and the Vole expects big things by expanding the franchise to Windows games too.”

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