First, congrats on getting a new machine! We've checked your license and you still have one of three seats available for installation. You can simply install Trend Micro Internet Security on the new tower using your existing license.
Also, here's a useful document that should be helped. You could check and manage the current status of your device installations. Managing devices under Trend Micro Account (MyAccount) - see "About Your License" of "Removing or Transferring protection from a device/computer"