On July 14 Microsoft will terminate extended support for Windows Server 2003. This means that there will be "no more patches fixing security vulnerabilities, non-security defects," etc. If Windows Server 2003 and the applications running on it play a critical role in your infrastructure, you need to start migrating to a new server platform now (if you haven't already).
While migration begins with the physical or virtual server and the operating system, it doesn't stop there. Ensuring a smooth upgrade path for applications will minimize downtime. It's important to understand the ramifications of migration up and down the stack, from operating system to database to application to Web front end. There are plenty of places to break the stack during migration, so careful planning and testing are requirements that need to be built into your project timeline.