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).