virtualbox - Guest CentOS 5.6 OS access folder on Windows 7 host

First enable host folder share via menu of VirtualBox window: "Devices->Shared Folders". I am sharing C:\ and "C_DRIVE" is automatically used as share folder name.
Then in guest OS, run the following command in a terminal:
sudo mount -t vboxsf C_DRIVE /mnt/host_c

The following command reported error:
sudo mount -t vboxsf //vboxsvr/C_DRIVE /mnt/host_c
/sbin/mount.vboxsf: mounting failed with the error: Protocol error