Code:
$stream = CreateObject("ADODB.Stream")
$stream.Open()
$stream.Type = 2
$stream.Charset = "utf-8"
$stream.LoadFromFile("C:\input.txt")
$text = $stream.ReadText()
$stream.Close()

$fso = CreateObject("Scripting.FileSystemObject")
$f = $fso.OpenTextFile("C:\output.txt", 2, 1, 0) ; -2 for Default, -1 for Unicode, 0 for ASCII
$f.Write($text)
$f.Close()