How to share a folder in VirtualBox

For WinXP:

• Add a shared folder using the VirtualBox interface (Details when it's turned off, or the shared folders icon in the bottom right when the VM is running), call it share, make it permanent.
• In the guest, open an explorer and go to Tools, Map Network Drive.
• Change the drive to whatever letter is convenient (I like to use Z:), change the folder to \\vboxsrv\share, make sure Reconnect at logon is checked.

You can now browse to the new drive and read and write files from it.