User Accounts
When you click "Add Icon", it adds an icon to the Control Panel through the registry key:
HKCR\CLSID\{98641F47-8C25-4936-BEE4-C2CE1298969D}
that has this icon:
%SystemRoot%\System32\Shell32.dll,111
and runs this command:
control userpasswords2
When you click on "Default / Remove", it simply deletes that registry key.
Network Connections (Vista)
When you click "Add Icon", it deletes "ncpa.cpl" from this key:
HKLM\Software\Microsoft\Windows\CurrentVersion\Control Panel\don't load"
When you click "Default / Remove", it adds the item back to the key.