Error when searching in Vim on Windows 7.

[SpikerTest]

On Windows 7, I installed ag from chocolatey, and added the path \chocolatey\lib\ag\tools\ into the PATH variable. Then I installed ag.vim plugin for Vim. However, when I try to search inside Vim by typing e.g. :Ag hello, it shows an error, saying that command ag not found, and asking me if silver searcher (ag) is installed and added to PATH. By the way, I can use ag in powershell. Could someone please help me? I just tried to start using Vim on Windows, but the first step has almost frustrated me...

[losingkeys]

You might need to open a new command prompt for path changes to take affect. So the command works outside of vim?

[SpikerTest]

The command works outside of vim, such as Windows Powershell. I tried restarting Vim and also Windows, it still cannot work in Vim.