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.
Now it comes the tough part: Which part of the Vista caused the problem? I've tried to uninstall/reinstall programs in my computer and used msconfig to disable startup programs. Zone Alarm was kind of suspicious since it was beta version (stable version was not available for Vista at that time), so I ran the uninstaller. That didn't help either. I have even tried to boot to the safe mode. None of which did any good.
After exhaustive diagnoses, and without getting the problem fixed, I decided to uninstalled my NIC (Network Interface Card) driver, which was an onboard Realtek Ethernet card. So I removed it from the Device Manager. A popup prompted me whether to delete the driver software for this device, I chose Yes. After uninstalling, I issued an "Scan for hardware changes" command to the Device Manager, it reinstalled my NIC. As soon as my NIC is installed, I tested my internet connection speed. It mysteriously went back to the normal (300kb/s).
Make sure Delete driver software is checked:
I have no idea what has happened, I guess it was a problem with the NIC driver. The important thing is the problem now is resolved. If you have similar problem and run out of solutions, it worth to give it a try.