I added some comments for each of problematic line,

> Set oXmlHttp = server.CreateObject("Microsoft.XMLHTTP")

-- antique progID, at least msxml2.xmlhttp

> oXmlHttp.open "GET", strurl, False

-- I suspect you want /post/ instead of /get/.

> '//open https connection to the server //
> oXmlHttp.setRequestHeader "Content-type", "text/xml"
> oXmlHttp.send oXMLDocument
>
> '// process the server response//
> iResponseStatus = oXMLHttp.status
> sResponseMsg = oXMLHttp.responseText

-- The last line should be placed on the exception block.

> If iResponseStatus = 200 Then '// success //
>      sResponseXML = oXMLHttp.responseXML.xml
>      'Below uses parser for XML 3.0
>      Set oXMLDoc = Server.CreateObject("MSxml2.domdocument.3.0")
>      oXMLDoc.loadXML(sResponseXML)

-- oxmldoc.load(oxmlhttp.responsebody) is better.

>      If oXMLDoc.hasChildNodes Then

-- if oxmldoc.load(oxmlhttp.responsebody) then

>           'Return drive/path from virtual directory
>           'e.g. /imaging/imghold/ path may be D:\imghold>           strSaveFile = Server.MapPath("/imaging/imghold/")
>           '    Make file name prefix be the userid
>           strSaveFile = strSaveFile & struserid & ".xml"
>           oXMLDoc.Save(strSaveFile)

-- The last line should come after you check the folder to be written. i.e. 
oxmldoc.loadxml("<a/>"): oxmldoc.save strSaveFile

>      End If


> Else
>    ...
> End If
>
>  Set xmlDoc = Server.CreateObject("Microsoft.xmldom")

-- antique progID. At least, msxml2.domdocument may get you msxml3.0.

>
>  xmlDoc.async = False
>  xmlDoc.load(strSaveFile)
> ...
>
>