In Windows Vista the networking components were designed to be much more intelligent and automatically adjust the network settings to optimize the connection speed. This is a great new feature. Your network components basically tweak themselves for optimal performance. In most situations, these components automatically adjust perfectly for best performance. However, with wireless WAN cards the detection of network conditions is flawed and can result in slower network performance.
To fix this problem, Microsoft has released a hotfix for users affected by this issue. If you have a Wireless WAN device, I suggest you give this hotfix a try.
KB 940646 details:
This problem occurs because of the way that TCP receive window auto tuning is used in Windows Vista for connections that only receive data.
Windows Vista obtains a round-trip time (RTT) estimate at the time of connection setup and every time that a new segment of data that is transmitted. A connection that only receives data is limited to the single RTT sample that is obtained at the time of connection setup. Because the connection only receives data, the connection cannot "converge" to the actual RTT of the connection. For example, because the RTT data may fluctuate for various reasons, a "converge" operation is performed to estimate a meaningful RTT by using blending current and previous RTT data. However, if the connection cannot converge to the actual RTT of the connection, the bandwidth delay product (BDP) estimate that Vista obtains is also incorrect. Therefore, the connection's receive window is limited, and throughput of the connected transmission network is reduced.