Share VirtualBox folders in reverse? Guest-Host?

There is no way to do this with the Guest extensions, however, setup normal Ubuntu file sharing on your guest and you can access the files using the virtual network between the host and the guest. The OSE version of VirtualBox doesn't have shared folders, and this is the recommended sharing method for VirtualBox as described here. How-To Geek has a tutorial here on how to do it with Ubuntu specifically, and it applies to everything outside of home folders as well.

The network driver is smart enough not to send the traffic over the wire, but will still be used to communicate with the guest. I have a few virtual machines I run and use like this, both on Mac and Windows.

By default, the virtual machine has a NAT connection to the local network, meaning it doesn't have a "real" IP address of its own. If you instead set up a bridged connection, you can use regular file sharing methods from the virtual PC, e.g. Windows file sharing (including SAMBA under Linux/Unix/etc.) or NFS. Of course the VM would have to be running.

It's also possible to mount a VDI as a drive under the host OS, I don't know the details of how to do that on an XP host, and I believe you would have to shut down (rather than suspend) the guest OS to avoid hard drive corruption.

There is a tricky solution:

1. In Windows, install cygwin
2. Use the same user names in both Windows and Linux
3. In Windows, create directory $HOME/shared and make it a shared folder in VBox
4. On your Linux create directories ~/shared and ~/shared_local and run: sudo /sbin/mount.vboxsf -o gid=1000,uid=1000 shared ~/shared
5. Use rsync to synchronize your data in ~/shared with ~/shared_local.

You can use the same bash scripts in both Linux and Windows. You can access any data in your ~/shared from Windows and from Linux using the same path.