Ho trovato questa guida(in inglese) su http://www.instructables.com/id/SPPFI14FF78QTXR/
step 1Whatcha need
To pull this off, you'll need:
A PC with a working dialup connection, running Windows XP (Should work on Vista as well)
An Ethernet Cable, long enough to reach your Xbox (For the uninitiated, these have plugs that look like bigger phone cord plugs)
And an Xbox 360.
http://www.instructables.com/files/deriv/FUT/4LX0/FF78QTZC/FUT4LX0FF78QTZC.MEDIUM.jpg
step 2Whatcha Do on the PC side
To pull it off, you need to connect the 360 to your PC using the Ethernet cable. Power up the 360 and the PC if they aren't already on. "Fire" up that dialup connection, and get ready to work the magic.
http://www.instructables.com/files/deriv/FCB/95AP/FF78QU0X/FCB95APFF78QU0X.MEDIUM.jpg
Open the Control Panel in Windows, then go to Network and Internet Connections.
Open Network Setup Wizard, press next.
Ignore the checklist and press next, then select "This computer connects directly to the Internet.".
Press next, and select your dialup connection as the way you connect to the Internet.
Press next again, and select "Local Area Connection" as your private connection.
Leave any names for your PC alone, and click next.
Leave the workgroup name alone, click Next.
Turn on file and printer sharing if you want to stream videos from your PC to the 360, click next.
Review the settings, and click next to apply them.
Let them apply, and select the last option that pops up, something like "I do not need to run Network Setup Wizard on the other computers."
Reboot your PC (then re-dial-up) if necessary.
step 3Whatcha Do on the 360 side
Now that your PC is configuratified, go to the System blade and select Network Settings, and select open "Test Xbox LIVE Connection". It will run a test on the cable connection, I.P. addresses, and the like. When it gets to the Xbox LIVE test, it will ask you to download updates if it's successful. Go find something else to do (not on your 360 or online on your PC) while they download.
If all goes well, you should be able to make your profile LIVE-enabled. I can't remember how I did it, but I'm sure you'll figure it out (or post how to in a comment for me to splice in).
http://www.instructables.com/files/deriv/F3D/M5AU/FF78QU1J/F3DM5AUFF78QU1J.MEDIUM.jpg
step 4Whatcha Can and Can't Do
Right, just because you're on Live doesn't mean you have all the features available to you that highspeed gamers will.
No multiplayer, with or without a Gold account. The lag will make it worthless.
No movie-on-demand. To be specific, a SD movie will take over a week of nonstop downloading.
But you can still:
Download Live arcade games and demos
Download game demos if you wanna wait that long
Update your console and games
Get themes, gamerpics and all that jazz
And you can still access MSN messenger.
http://www.instructables.com/files/deriv/FVQ/B14D/FF78QU1P/FVQB14DFF78QU1P.MEDIUM.jpg
step 5Whatcha don't need to comment about
If you're reading this saying to yourself, "Self, this is useless", or "Self, these guys need to get off their n00b-asses and pay for highspeed", please refrain from commenting. This is for the people who for one reason or another cannot access highspeed internet, yet wish to update their console or get some XBLA games. It's hard enough for a 56K pimp without people trash-talking.
So if you thought this was useful or have questions, don't refrain from commenting, or adding me to your Live friends list. If the photo didn't tip you off, my GT is Radical Jake.
http://www.instructables.com/files/deriv/FA4/9BI4/FF78QU1I/FA49BI4FF78QU1I.MEDIUM.jpg
con questo tipo di connessione nn sarà possibile giocare senza rallentamenti...anzi praticamente si potranno giocare pochissimi giochi...ma almeno gli aggiornamenti e alcuni contenuti potranno essere comunque scaricati...scusate l'ignoranza ma non capisco come fa anche se continuo a guardare la guida...cmq grazie all'autore