The most likely explanation is that you have some sort of IRC virus. This means that when someone joins a channel, you message them with some sort of link for free pornography, or a promise a picture of yourself etc, when actually the link contains an exploit that will infect other people's computers.
The "floodbot" is also called a GTBot - or Global Thread Bot - and they are used to attack users, flood channels and attack servers. Quite a big problem on the Internet would be removed if nobody in the world was infected with a virus
- Unfortunately, people are out to trick and deceive you everywhere you go, it's a bit like playing some sort of video game nowadays. You just got caught out I'm afraid!
However, fear not! There are a huge amount of good guys to help you out and they've invented a large number of virus scanners etc that scan your computer and delete or quarantine (isolate) the virus threat. This thread
will help you stay safe in the future, and also suggests some ideas for cleaning viruses you have right now. I strongly suggest you read the entire thread and don't just skip down to the links.
There is a channel on DALnet called #NoHack and those guys came up with a script to remove quite a few IRC virus threats. I would download their script here
and try it out (That is a direct download link). I suggest this separately because some IRC viruses are more annoyances than dangerous to your harddrive or anything, and so some antiviruses such as Norton may not be set to detect them. Again, make sure you read the instructions to the letter.
I suggest you run 2-3 virus scanners, as no ONE can detect everything. Almost all AntiViruses will have an 'update' feature so you can stay safe from the latest viruses. Before you scan you should use the update feature to get any updates for the software before scanning. You should also definitely close mIRC and any other running applications, even ones in the taskbar, MSN etc.
G'luck & stay safe!