You could also do it using VBScript. Here's an example (based on this script):
Set objFSO = CreateObject("Scripting.FileSystemObject")
strFolder = CreateObject("Scripting.FileSystemObject").GetAbsolutePathName(".") ''# Current directory
Set objFolder = objFSO.GetFolder(strFolder)
For Each strFiles In objFolder.Files
If objFSO.GetExtensionName(strFiles) = "html" Then
strComponents = Split(strFiles.Name, "_")
strFiles.Name = strComponents(0) + ".html"
End If
Next
Save the above as something.vbs in the directory it is to run and double click to run it.