virtualbox.org

I have been able to share a folder in my host OS with my guest, but now I would like to do the reverse...

That is have a folder in my Guest OS be able to be "mapped" in my host OS.

Host OS: Windows Vista Ultimate 32,
Guest OS: Windows XP

Only with a Bridge (Host Interface) and Windows Shares!

Bridge is not neccesary, HIF is. Create one Host Interface, attach it to the Guest and set an IP address on both ends in the same range. Enable sharing on the Guest and you can do what you want.