How to register a Dll for Delphi in Windows XP?

Question

I need to register a dll by Delphi (xe2) in Windows XP, but when running the windows method opens a "Run as" window where you need to indicate the user who will perform the action. Even though my user is an administrator on the machine.

I used the following method to record the Dll:

procedure TForm1.ExecAdmin(Programa, Parametros: string);
var sei: TShellExecuteInfo;
begin

  ZeroMemory(@sei, SizeOf(sei));

  sei.cbSize := SizeOf(TShellExecuteInfo);
  sei.Wnd := Application.Handle;
  sei.fMask := SEE_MASK_FLAG_DDEWAIT or SEE_MASK_FLAG_NO_UI;
  sei.lpVerb := PChar('runas');
  sei.lpFile := PChar(Programa);

  if Parametros <> '' then
    sei.lpParameters := PChar(Parametros);

  sei.nShow := SW_SHOWNORMAL;

  ShellExecuteEx(@sei);

end;

The call of the method:

ExecAdmin('regsvr32.exe', '/s c:/sistema/msxml5.dll');

Doubts:

a) Need administrator privilege to register the dll? b) How to execute the method silently without the user check box appearing?