I first introduced this issue in a previous post. It turns out that this problem occurs if a network hardware device, such as a router, does not support TCP Window Scaling. TCP Window Scaling is a new Windows Vista feature.
The TCP window scale option is an option to increase the TCP congestion window size above its maximum value of 65,536 bytes.
I also have found that this can resolve problems like slow file copy operations in Vista as well as momentary loss of Internet connectivity.
Here are a couple of knowledge base articles from Microsoft: