I'll add that OneCare allows Windows Media Player by default unless you chose to block it due to being prompted. If it is unable to access the Internet/network, but turning off the OneCare firewall temporarily allows it *and* you do not see Windows media Player in the block/allow list as blocked, please contact support.
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
-steve
Microsoft MVP Windows Live / Windows Live OneCare & Live Mesh Forum Moderator