var
 buf:array[0..255] of char;
 p:integer;
 s:string;
begin
 s:='%systemroot%\file.txt';
 GetWindowsDirectory(buf,255);
 p:=pos('%systemroot%',s);
 if p>0 then
  begin
   delete(s,p,12);
   insert(buf,s,p);
  end;
 showmessage(s)
end;