Although it may appear that disabling IPv6 in Windows Vista and Windows Server 2008 is only a matter of clicking a checkbox, doing this may result in some undesirable results. At first inclination, it would seem that IPv6 can be disabled in the properties of the network adapter you are concerned about. However, in order to properly disable IPv6 you must edit the registry.
In order to do this, run regedit and browse to HKLM\SYSTEM\CurrentControlSet\Services\Tcpip6\Parameters\. The key you want to edit is called DisabledComponents. In order to disabled all IPv6 components, you must give this key a value of 0xffffffff. The default value for this key is 0, which enables all IPv6 components.
As always, ensure that you make a full registry backup before making any changes.