Since you are attempting to use a nick that is already in use, then you have to pick another nick that:
A) Is not in use.
B) Is not registered on the network.
I don't recommend adding a bunch of numbers to the end of your nick to make it unique, as this is a common method of giving bots unique nicks, and may make the network think that you are a bot.
In most cases nicks are not unique based on capitalization of the nick, but most networks do allow at least 15 characters for a nick.
mIRC is not saying that your system is infected, however, the security system on the network you are attempting to use is saying that you have been recognized (probably via IP address) as being infected. Unless you authorized the network to download and install a program on your computer, the system is, probably, assuming that you are infected, due to your IP address matching an infected range of IP addresses.
My suggestion, is to contact the network that you wanted to connect to, via their website (presuming they have one), and simply explain to them that you have run the scans and your system is showing as clean.
If you can't contact them via a website, you can always try a different network (there are hundreds of possibilities out there).