It's a holdover from the time before the Berne convention where you needed to explicitly state what rights you were reserving when copyrighting a work. The US entered the Berne convention in 1988. Before that the Universal Copyright Convention (1952) was in force, which required a copyright notice and registration at the copyright office. Under the Berne convention (and the later WTO TRIPS agreement) neither of these things are required, so the entire "Copyright (c) Microsoft Corporation. All rights reserved." line is redundant. As you note, it merely states something that is already true. But it's become traditional to include something like that, often including the date, sometimes using the © symbol, etc.