Microsoft made Vista (and Windows Server 2008) IPv6 compliant, which is a proper thing to do given the "shelf life" of Vista. There is a side-effect to this, and if you are experiencing a "general slowdown" of your Internet connection while using Vista, you might consider turning off the newer, and sparsely used IPv6 protocol for your network connection. Our network connection is not IPv6 compliant, and very few are right now.

In general terms, every Internet request (DNS query) and such is sent in in both IPv4 and IPv6 from the Vista machine. That's sort of like driving up to the take out Window and ordering twice. The first time (IPv4) asking for the #2 and a diet drink. The the second time (IPv6) getting very specific about your order and while they prepare your meal to order, you might have to pull into a parking space and wait longer for it to be delivered.

It might not fix all your Vista woes, but it might give your Internet a caffeine jolt and more "normal".

From Microsoft at:

http://www.microsoft.com/technet/community/columns/cableguy/cg1005.mspx#ENAAC 

Unlike Windows XP, IPv6 in Windows Vista and Windows Server 2008 cannot be uninstalled. To disable IPv6 on a specific connection, you can do the following:

In the Network Connections folder, obtain properties of the connection and clear the check box next to the Internet Protocol version 6 (TCP/IPv6) component in the list under This connection uses the following items. This method disables IPv6 on your LAN interfaces and connections, but does not disable IPv6 on tunnel interfaces or the IPv6 loopback interface.

Here's a more "geeky" explanation for those who want it.

http://news.com.com/Will+Vista+stall+Net+traffic/2100-1016_3-6112338.html