Your mIRC registration has nothing to do with whether you can use /nick or not. I suggest you type /remote off and see if you have the /nick command aliased - try //echo -a $isalias(nick) - if it says $true, I suggest you delere it with /alias nick.
Guest nicknames are also given when you try using a registered nickname and if you don't identify to it after what is usually 60 seconds then you get forced to use a Guest nickname.
What is FC Guest by the way? Did you download mIRC from the official mIRC website only (i.e. this one)? Was installation successful? Are you using a non-edited copy?
Regards,
