IPv6 for Windows
From KlavoWiki
Unlike Windows XP and Windows Server 2003, IPv6 in Windows Vista and Windows Server 2008 cannot be uninstalled. However, you can disable IPv6 in Windows Vista and Windows Server 2008 by doing one of the following:
Add the following registry DisabledComponents value (DWORD type) set to 0xFF:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip6\Parameters
This method disables IPv6 on all your LAN interfaces, connections, and tunnel interfaces but does not disable the IPv6 loopback interface. You must restart the computer for this registry value to take effect.