I never had to register mirc until I got the newest version (6.12) which has that nag screen with the piccy of the dodgy looking fella on it
Froz you probably have an older version but I heard there were some security issues so it might be a good idea to upgrade.
I also use invision, never had any problems with it although a lot of the features don't seem to work properly (channel protection etc.) The main bonus is that it looks a million times nicer than normal mirc... gets rid of the commodore 64 look and gives you white on black text to avoid burning your eyes out like normal mirc
