Before use this code 'net use x: \\vboxsvr\name_of_shared_folder' at Windows Command Prompt you must:
01. create the shared folder at 'Devices\shared folders' menu from VirtualBox
02. install from the same menu 'Devices' the 'guest additions' - My VirtualBox is in portuguese so I'm translating the name, but is the last item at 'Devices" menu. This will start an install process inside your windows virtual machine.
03.finally, go to the prompt inside windows and type the command 'net use x: \\vboxsvr\name_of_shared_folder'
YOUR VIRTUAL MACHINE MUST BE RUNNING DURING THIS PROCEDURE
I test this with MacOS 10.5.6 (iDeneb) as host and Windows XP as guest