Help in setting up file access to networked computers

I am setting up a new Vista Home Ultimate computer. It is networked with a
XP Pro and Windows Media Edition machines.

They are all in the same Workgroup. The XP and WME have full access to each
others files over the network.

For the life of me, I have not figured out how to give them access to the
VISTA and visa versa. I can "see" all of the machines from each machine.

I understand that I can not move the "Public" file directory to another
drive.
I have too many large files to put on my system drive.

I have tried to set the shares on all of my directories and drives to share
... but that has made no difference.

I have been doing this as the administrator..

I would like to block access to computers that are not members of my
workkgroup

Any suggestions, preferably in step by step directions?

Thanks

Dunc