The folks over at gamersreports has a how to : Browse The Internet On Your Xbox360.

You NEED WindowsXP Media Center Edition, it will not work with Windows 2000 or any other form of Windows XP

1) Go to http://www.anpark.com/software.aspx
2) Click on "Download Now (591 Kb)"
3) Double click on MCEBrowserSetup.msi and just click next
4) Start your Xbox360 and go into Media Center
5) Go into "More Programs" and choose the "Browser" Entry.
6) Add Favorites to your 360 from your PC, by going into:

C:, then "Documents and Settings" folder, then the "MCX#" folder

(# is a number, it depends on how many times you did manually reconnect the 360 to the PC , so it changes for everyone. The highest number is the latest, so choose that folder)

Go into the Subfolder called "Favorites" and either add links by drag'n'drop from Internet Explorers Address Bar to it, or right click and choose "Create Shortcut", enter the URL, click next, and now Enter the Name of the Website (This will be how it appears on the 360)

7) Choose One of your "Favorites" on the 360 and enjoy!