the shared folder doesn’t appear just like that (unfortunately). You have to use the “net use K: \vboxsvr<name of shared folder>” command. If you hover the mouse over your virtual box settings -> shared folders settings (where you can add remove edit folders), you can see in a text box below that you have to use this command.
You can also use samba on the Linux to setup a network. Then you can use regular network operations to brows XP from Linux and Linux from XP just like any network.
The addons must be installed in VBox for the shared folder thing to work. You also have to move up to look at the entire network to see the connection it is NOT in your Mynetwork folder.