If a managed resource gets a new OS, the machine will not automatically populate in the default filter "Windows Computers with no Symantec Management Agent Installed." This happens because the database already has a record of that machine ID and has it listed as managed. Therefore, the resource will not get a newly installed agent automatically.
To ensure the managed resource is removed from the database, you will need to do one of the following:
Once the resource has been rediscovered, you can use the "Symantec Managment Agent Install" (Formerly the "Altiris Agent Install") Policy to reinstall the agent.