Did you recent install any sort of browser plug-in when this problem started? Is it only the web browser or all Internet access such as email?
OneCare should not be blocking access to the Internet, of course. If disabling the firewall resolves it, and enabling the firewall blocks access again, I recommend that you 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|MS Security Essentials Forums Moderator ~