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.
read more at
http://www.giannistsakiris.com/index.php/2007/09/28/virtualbox-access-shared-folders-from-windows-xp-guest-os/