A. Windows 2003 R2 is Windows 2003 SP1 plus optionally installed components. The program comes on two CD-ROMs: The first CD-ROM is just Windows 2003 with SP1 integrated into it; the second CD-ROM includes the new R2 content and a module to update the Add/Remove Windows Components section of the Add or Remove Programs Control Panel applet with the options to install the new R2 features, an updated "Configure your Server" Wizard, and Microsoft Management Console (MMC) 3.0.
To install R2 on an existing Windows 2003 SP1 server, you need only the second R2 disk (or R2D2 :-) ). The disk should autostart and display the option to Continue Windows Server 2003 R2 Setup, which will prompt you for a new R2 product key. (You can't use your Windows 2003 product key for R2.)
Alternatively, you can install R2 via the command line and pass the product key as a parameter to setup2.exe (which is responsible for the R2 part of the installation) using /productkey: or /p: with format xxxxx-xxxxx-xxxxx-xxxxx-xxxx. If the product key is passed, you must also use the /quiet switch to eliminate prompts during the installation, and the /a switch to automatically accept the End User License Agreement (EULA), as the example below shows: