First, let's confirm it is the OneCare firewall that is the issue.
Open OneCare, click Change Settings and temporarily disable the OneCare firewall. Can you share the files? If so, let's turn the firewall back on and then go to the Advanced button on the firewall tab.
Click on the Network Connections tab. Is your active network connection shown as "Home or Work?" If it shows as Public, click Change Location.
Click on the ports and protocols tab.
Scroll to the bottom of the list and place a check mark next to the xbox 360 entries. Scroll back up and make sure that File and Printer Sharing for local network is checked. You can optionally place a check mark next to the other File and Printer Sharing item and you may need to do this if the PS3 or the PC are wireless and the other is wired.
Click OK and try the media sharing again,
If this still doesn't work, please contact support.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve
