Vista - Remove Standard User from File Sharing

I have a new PC with Windows Vista Home Premium where I set up an "Admin"
account (which I use) and 3 Standard User Accounts for my wife and 2 kids. I
am using the "Admin" account for my files.
Somehow, the system is sharing all of my files (Admin account) with my
wife's Standard User account but not the other two accounts. This is neither
desireable nor did I INTEND for that access to be granted.
In Windows Explorer, if I open the "C" drive and single click the "Users"
folder in the RH pane, the lower Status/Properties bar says:

Users Date Modified: 3/4/2007 7:01PM
File Folder Shared with: Administrators;Authenticated Users

Opening the "Users" folder I see, in the RH pane, the 4 folders of the
accounts that I created and the "Public" folder.

If I single click on any of the User folders in the RH pane, the lower
Status/Properties bar just has the folder name, the words "File Folder" and
the Date Modified fields. It doesn't show anything about "Shared with:" at
all for any of the User accounts.

Now, if I open my User Account ("Admin") and single click on ANY of my file
folders in the RH pane, the Status/Properties bar shows (for example):

Contacts Date Modified: 3/4/2007 4:33PM
File Folder Shared with: Smith-Home-PC/Melissa

where "Melissa" is my wife's account. I have no idea why my WIFE's account
but not my two kids accounts appears in the list because i never manually
added her. (At least not purposefully or consciously)

So, I right click on the "Contacts" folder and then on "Share..." A window
pops up that says "This folder is already shared." and gives me the options
of "Change sharing permissions" or "Stop sharing." If I click "Change
sharing permissions" Windows displays another dialog box "Choose people to
share with" and I see myself listed as "Owner" and Melissa as "Reader." I
click the down arrow by Melisas's "Reader" status and have the option to
"Remove." I click that and her name dissappears from the list. I click
"Share" button (the only alternative to "cancel") and wait an eternity while
the hard disk trashes away for, literally, 5 minutes.

Finally, a window appears stating "You folder could not be shared." and
"Click here for more info..." Clicking the "more info" link, I get,

Some tiems could not be shared
Contacts
C:\Users\Admin\Contacts
This folder is in a shared folder.

I've also tried the "Stop Sharing" command in the original dialog box but I
get the same thing.

Any help?