To expand on Wenske's reply - open OneCare, click on change setting. On the firewall tab, click on
Configure Firewall and make sure that it is shown as Home or Work and *not* Public.
Then, click on Advanced Settings and the ports and protocols tab. Scroll down to find the entry for xbox360 and media center extenders. Place a check next to that entry and also next to the xbox 360 and network media players entry.
Click OK to save the settings.
-steve