Watch out for IPv6 on Windows. I have a PPTP VPN through StrongVPN and the IPv6 interface is unaffected. Unfortunately there doesn't seem to be a way to programmatically disable it.
I was going to post this link for disabling ipv6 http://support.microsoft.com/kb/929852/en-us
then I read the word "programmatically"..
then I was going to suggest using "netsh" but it seems you can't use it to disable IPv6, finally I found that you can modify the registry as illustrated here:
http://technet.microsoft.com/en-us/library/bb878057.aspx
(i.e. with a script launching reg.exe) but then I read that it should require reboot (damn Microsoft..)
