Windows XP with IE 7
A separate W2K3 machine works with all four addresses.
I did the same IPv6 installation on both machines (Network properties>Install IPv6)
Check the bindings is IIS or which ever web server you are using locally. Otherwise the port isn't going to be listened to.
If you're using IIS 5.1, it doesn't support IPv6, so it won't bind to the IPv6 addresses and ports. MS added the support in 6.0.