Vista - If you really want to disable IPv6, you can.

There are fine grained controls if you want, but to disable IPv6 without yanking out support overall, globally, you can set:

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip6\Parameters]
"DisabledComponents"=dword:000000ff

and if you want to back out that change, either delete 'DisabledComponents" or set it to 0.

More information about it here and here.

If you are curious what each bit of the DWORD does, here you go:

Disable all tunnel interfaces:
*0x1*

Disable 6to4:
*0x2*

Disable ISATAP:
*0x4*

Disable Teredo:
*0x8*

Disable Teredo and 6to4:
*0xA*

Disable all LAN and PPP interfaces:
*0x10*

Disable all LAN, PPP, and tunnel interfaces:
*0x11*

Prefer IPv4 over IPv6:
*0x20*

Disable IPv6 over all interfaces and prefer IPv4 to IPv6:
*0xFF*

An interesting sidenote, the codename of Teredo has an interesting background - a shipworm background.