"ACK starvation" could happen when the download speed is much higher than the upload speed. Generally a ratio greater that 10:1 can lead to such a problem.
The first thing to check would be your MTU, that can be done from a command prompt using the "ping -f somesite" command (replacing "somesite" with some known to work server). If that fails, you MTU is too high and IP packets are being fragmented (doubling the number of packets that have to be sent).
The later Windows versions are much better on the second possible reason, an RWIN that is too low. RWIN is the number of packets that will be sent from a server you are downloading from before it expects an ACK. Generally, the higher you download speed, the higher your RWIN can be.