What this often means is that your nick and/or ident look suspicious. Many networks have certain patterns for nicks and idents that they automatically ban because trojans/bots/etc use them to spam or infect others. For example, an ident of "dfghj" can get you banned from certain networks. And, although not automatically banned by the network, Undernet is filled with porn bots posing as a female's first name followed by two numbers, such as "Mary14". Other networks might automatically ban something like that if it became as much of an issue as it is on Undernet. On Undernet, many channels just ban those users themselves since Undernet doesn't.
So... try changing your nick and/or your ident. Alt-O > Connect > Identd for changing the ident. Note that if you have the checkbox enabled for using e-mail as ident, you'll need to either uncheck that box and fill in what you want there for your ident, or go and change the first part of your e-mail in mIRC (the part before the @).
