If you're running Windows Server 2003 on production systems, you need to etch the date July 14, 2015 into your brain, because that's when Microsoft will terminate extended support for the venerable OS. That date is rapidly approaching, so it's time to get moving on your migration plans. The more complex your operation is and the more heavily you rely on Windows Server 2003 to run production systems, the more time and effort you're going to have to put into migration. It's possible that your operation will require three or more months to make the switch, which means that you'd better get on this now.
Migrating from Windows Server 2003 is critically important as extended support for Standard, Enterprise and Data Center (32-bit and 64-bit) will end. This means that there will be no more patches of security vulnerabilities or non-security defects or operational issues. Moreover, third-party products such as business applications will not be supported on Windows Server 2003. No third party could guarantee that their software would continue to run on unsupported systems, because defects might arise in the underlying OS that can't be addressed.