Once you move the PCs out of the local LAN environment, you can no longer share folders. That isn't a OneCare issue, it is a Windows File Sharing issue. You may want to look at a product such as Windows Live Skydrive (free, files shared via a web site, folders can be truly public or shared with specific people or only available to you from multiple computers) or Windows Live FolderShare (also free, in beta, requires the installation of client software to share specific folders on each PC with the client software installed, using the subscribed LiveID) to share files between PCs located anywhere in the world, still offering security for that sharing.
-steve