Uses
  XMLDoc, XMLIntf;

function Transform(XMLContent : string; XSLContent : string) : WideString;
var
  XML : IXMLDocument;
  XSL : IXMLDocument;
begin
  XML := LoadXMLData(XMLContent);
  XSL := LoadXMLData(XSLContent);

  XML.DocumentElement.TransformNode(XSL.DocumentElement, Result)
end;

uses
  MSXML2_TLB;

function XMLTransform(const xmlFile: string; const xslFile: string): WideString;
var
  XMLDoc, XSLDoc: IXMLDOMDocument2;
  xslTemplate : IXSLTemplate;
  xslProcessor : IXSLProcessor;
begin
  XMLDoc := CoDOMDocument40.Create;
  XMLDoc.async := False;
  XMLDoc.load(xmlFile);

  XSLDoc := CoFreeThreadedDOMDocument40.Create;
  XSLDoc.async := False;
  XSLDoc.load(xslFile);
  
  xslTemplate := CoXSLTemplate40.Create;
  xslTemplate.stylesheet := XSLDoc;

  xslProcessor := xslTemplate.createProcessor;
  xslProcessor.input := XMLDoc;
  xslProcessor.transform;
  result := xslProcessor.output;
end;