IPv6 is the latest, greatest, cutting edge version of the IP protocol. It is designed to be more secure, more efficient, more stable, and provide a larger (exponentially larger) pool of addresses to work with than its IPv4 predecessor. So, why would you want to disable it? In this SearchNetworking.com article, Brien Posey provides additional reasons, but the primary one is system resources. Windows Vista runs both IPv4 and IPv6 functionality simultaneously. If your network infrastructure is not actually leveraging the advantages of IPv6, then precious Windows Vista system resources are being eaten up by a protocol you are not even using. Of course, Posey also provides some logic behind why you would want to leave IPv6 on. Take a look at Disabling IPv6 in Windows Vista — Pros and cons to read the complete article and decide for yourself if you should disable IPv6 in Windows Vista.