In your VirtualBox virtual machine parameters, there is the last item called
"Shared Folder". Just select a local folder that you have write access and give
it a shared name. Create the share and select it for your machine. Then
launch your virtual XP machine, open a dos shell and type that command :

net use x: \\vboxsrv\YourSharedFolder

And voilà! You have mounted your shared folder. You may specify, in the
parameters, that you want an automatic mount of the folder I think.

Don't forget that you need the Extensions for that.

I can't tell you much about it since I compiled VirtualBox with extensions
support (I'm on Funtooo/Gentoo) and I don't need to add them post-installation.