You are reading the response in the wrong way. Here is an example, the helpfile contains some others.
#include<WinHttp.au3>
$hInternet = _WinHttpOpen()
$hConnect = _WinHttpConnect($hInternet, 'snee.com')
$sFirstName = "Jack"
$sLastName = "Berk"
$sSomeOtherURLEncodedVariables = "&a=123&test=void&foo=bar"
$sPost = "fname=" & $sFirstName & "&lname=" & $sLastName & $sSomeOtherURLEncodedVariables
$sResult = _WinHttpSimpleRequest($hConnect, "POST", '/xml/crud/posttest.cgi', '', $sPost)
_WinHttpCloseHandle($hConnect)
_WinHttpCloseHandle($hInternet)
MsgBox(0, '', $sResult)