From the VirtualBox's menu, go to Devices → Shared Folders... to add/remove the folders of the host operating system that you would like to share with the guest operating system (Windows XP, in this example). When done click Ok.
Now we are going to access the folders we added. DO NOT open My Network Places! Instead open a new explorer instance. You can do that by pressing WinKey-E or by executing the explorer command (Start → Run... → type "explorer" → Ok). You should see a windows that looks like this:
Now, carefully click on the expand icon () right next to My Network Places. Make sure you click exactly on the expand icon. If you accidentally clicked on My Network Places, you ruined everything and you will have to go back and start all over again. After expanding the My Network Places branch, you should be able to see Entire Network right underneath:
Now expand the Entire Network branch, too, and you should be able to see VirtualBox Shared Folders:
Open VirtualBox Shared Folders. Normally, you should be able to see your shared folders in there.