As you can see in the support matrix the OneAgent does not support Windows Server 2003.
There is an ActiveGate extension available to bring in the most important metrics about the health of windows systems where you cannot install a OneAgent:
If you're interested in it you can reach out to Dynatrace One or your account team. Please note that it will not give you deep monitoring into the processes.
If you have appmon on your envrionment as well it is possible to setup older version of collector, deploy old version of agent. After that it should be possible to link Dynatrace and appmon together (I'm not sure if this feature needs only appmon server on 7.2 version or agents as well). In such case you can have links on purepaths between systems. Unfortunately it needs separate appmon license so it is not available for all.