function GetComputerName: String;
var
  Buffer: Array[0..MAX_COMPUTERNAME_LENGTH+1] of Char;
  Size: DWORD;
begin
  size:=1024;
  Windows.GetComputerName(Buffer, Size);
  Result:=StrPas(Buffer);
end;
 
procedure TForm1.Button1Click(Sender: TObject);
begin
  Caption:=GetComputerName;
end;