The Windows Automatic Updates program connects with the Microsoft Update website to downloads all the hotfixes, security patches and other recommended updates for Windows, Internet Explorer and Microsoft Office programs after validation.
On the other hand, the Microsoft Download Center is the place for downloading new software that have just left the Microsoft stable. This could be a new version of Windows Defender, Games, Windows Live Messenger or some new PowerToy for Tablet PCs.
Though Microsoft provides e-mail notifications for new software downloads posted on Microsoft Download Center, it's a bit strange that they don't offer RSS feeds for the same.
You can however subscribe to this third-party Feedburner feed at MicrosoftDownloadCenter to keep yourself updated. Not an official feed from Microsoft but always in sync with the Microsoft website.
Highly recommended if you are on the Windows platform.