Can't Share The Windows Vista C Drive - Why?

Folks,

I have finally got my two Windows XP PCs and my new Windows Vista PC to
network just fine. Sharing amongst the PCs is working well. However, there
is one annoyance. I have set my Window Vista PC’s C Drive to advance sharing
and it appears to accept this. However, when I attempt to access the C-drive
share from my WXP machines, it states I don't have permission to bring it up,
etc. I did the same advanced share process on an external hard drive
(Iomega) attached to the Vista PC and at the Windows XP PCs, it comes up just
fine for viewing and manipulation. What am I missing here? Permissions have
been granted to be full for that C-Drive. Any help would be appreciated.