Microsoft Knowledge Base Article 897341 contains the following INTRODUCTION:
This step-by-step article describes how to determine whether a security update supports HotPatching, how to set up an environment to test HotPatching, and how to install security updates that use HotPatching.
If you use HotPatching, you can install General Distribution Release (GDR) security updates on servers that run 32-bit versions of Microsoft Windows Server 2003 Service Pack 1 (SP1), without restarting the servers.
Note Not all security updates support HotPatching, and some security updates that support HotPatching might require that you restart the server after you install the security updates. Before you use HotPatching to deploy a security update in a production environment, you must determine whether the security update that you want to install supports HotPatching and you must evaluate the security update installation in a comparable test environment.