Never mind, I have found it myself.
If someone is interested here is the code, taken from Torry's Delphi page.
The Autor is : Simon Grossenbacher


{ Retrieve the computer name }

function GetComputerName: string;
var
buffer: array[0..MAX_COMPUTERNAME_LENGTH + 1] of Char;
Size: Cardinal;
begin
Size := MAX_COMPUTERNAME_LENGTH + 1;
Windows.GetComputerName(@buffer, Size);
Result := StrPas(buffer);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
ShowMessage(GetComputerName);
end;

Ciao

Arnaldo