ArcServe IT for Windows NT 4.0

Can someone tell me why is it that when I look under the source tab and select the computer on the network that I wish to back up, it shows the following:

C$
D$
{a directory name}
D (which lists the same thing as D$)
{another directory}
{another directory}
{another directory}
Registry

I don’t understand why there are two separate listings for the D drive. I realize that D$ is the default administrative share, but why isn’t there one for the C drive too?

Also, why are certain directory names specifically listed while others aren’t?