Sharing folders can be done in many ways. Windows 7 is sharing media files via UPnP which the N900 handles out of the box. On Ubuntu, folders are shared via either SMB or NFS, which are both supported on the N900, but don't work out of the box (see the wizardmounter app for accessing SMB or NFS shares).
You can also run an UPnP server on Ubuntu for sharing media (e.g. fuppes, MediaTomb, Rhythmbox with plugin, etc.).