confused about sharing

I have 2 computers with XP home thru a wan router connection. They see
each other and I can share some files ( not all )....IE..I can't share
my documents nor program files within that folder. I highlite it and
clik on the menu that appears, "Sharing and security". When I go there,
a box appears again and most if not all boxes are cleared and greyed.
It does say that it has to dragged to a shared folder. I don't get it.
Could someone clarify this for me? Thanks