Note: This was done using Ubuntu 7.10 Gutsy as the host and Win XP as the guest.
1. Ensure you are not running the OSE version of VirtualBox.
2. Start your virtual machine and click Devices > Install Guest Additions. If nothing happens click Devices > Unmount CD/DVD-rom, then try again.
3. Click Devices > Shared folders
4. Click the add button and on the next screen and enter the path of the shared folder I.e. "/home/graham/whatever" (without the quotes|) and then add in the name for the shared folder. This name you have just given the folder is the important part.
5. Open a DOS command prompt and enter the following
net use z: \\vboxsvr\whatever
The "whatever" part is the name that you gave the shared folder while specifying the path.
After this the Z drive or whichever drive letter you assigned it to should appear under "My Computer".