Windows XP (Host OS) and Ubuntu 11.04 (Guest OS).
1. Open VM and go to tab Devices -> Shared Folders
2. In the right hand side find the add folder button and click it
3. In the folder path filed select other and browse your folder that you want to share
4. Then then check Auto-mount and Make Permanent and hit ok
5. open the Terminal from ubuntu and write the following one command
sudo mount -t vboxsf shared_folder-name_in_your_windows full_path_of_ubuntu_share_folder