Windows Sharing files over the internet and accessing them from another location

If you're wanting XBMC to stream in the content from a remote media server over the internet give this a go seems to work pretty well -
http://www.youtube.com/watch?v=gRYWh4ISf5w

If your after remote file sharing I'd say FTP for remote file access but make sure you set up the ftp software on the server to use FXP, it allows you to move files and folders around on the server without it downloading to the client pc and then re-uploading again.

More info about FXP
http://en.wikipedia.org/wiki/File_eXchange_Protocol

Good luck!