If you did a Portable install of mIRC, the registration information will be stored in mirc.ini instead of your system registry. In this way, the Portable install can be easily moved from one system to another.
You can alternately attempt to copy the Windows Registry information for mIRC from the old computer to the new, but I can't guarantee this will work, and you can risk damage to your system registry. Proceed with caution.
Try to recover your registration on the following webpage. Try each and all of the possible email addresses you own.https://www.mirc.com/reglost.html