Unfortunately, mIRC is used in certain trojans to connect your computer to IRC and then participate in Denial of Service attacks against people, websites and/or ISPs.

You need to scan you computer with multiple virus scanners, also use an online virus scanner such as Trendmicro. Lots of links can be found in this thread. You might also wish to run spyware programs.

Search your computer for 'mIRC' and delete the mIRC.exe file. (Use Start > Search).