Have you set up shared folders and installed the vbox tools on the guest? It seems you're trying to access the host C: drive via the "normal" CIFS network share. It's much faster, and harder to break, if you go with shared folders instead:
Boot the Guest and open a terminal.
Create a folder where you will mount
it on. E.g. in your home folder. I
~/host for the mount point.
Now mount it with the following
sudo mount -t vboxsf share ~/host
Note that with this, the default mount
options are used and all files are
owned by root.
This can be changed by adding some
mount options. Options are passed on
with the -o parameter. You can use
multiple options with one parameter,
seperate the values with a comma. See
the man page of mount for more info on
which options you can use. The User
Manual also notes the options
compatible with the Shared Folders. To
mount the SF so that you are the owner
of the files, use this command:
sudo mount -t vboxsf -o uid=1000,gid=1000 share ~/host