The download speed on my Windows Vista have dropped from 300kb/s to around 20kb/s. It took me several attempts to find out what the problem was.
It first seemed to me that my ISP was in a technical difficulty state. I am on a dual-boot system with both XP and Vista. In order to verify that Vista was not the cause of the problem, I booted into Xp and did some speed throughput test. Surprisingly my download speed in XP was normal (300kb/s). Clearly the cause for my slow download in Vista was the Vista itself.