What is SMS Agent Host?

As you mentioned, this is the agent for Microsoft SMS. SMS can be used to collect inventory, apply patches, deploy software, etc. This is not something that is specific to Server 2003/Windows XP x64. This shouldn't be on a machine unless you have SMS deployed. You can stop the service if it's killing a machine, it just won't report back.

The newer versions of SMS are called System Center Configuration Manager.