You cannot "reinstall" .NET Framework v1.1 on Windows Server 2003, because
it is a core component of the operating system.
You can reapply .NET Framework v1.1 Service Pack 1 on a Windows Server 2003
RTM system, or you can reapply Windows Server 2003 Service Pack 1 (which
contains .NET Framework v1.1 Service Pack 1).
If you believe the core .NET Framework v1.1 is damaged on a Windows Server
2003 RTM system, you should perform a 'repair' installation of Windows
Server 2003; or, alternatively, perhaps a better option, apply the .NET
Framework v1.1 Service Pack 1, or even better, apply Windows Server 2003
Service Pack 1.