mIRC as downloaded from an approved site found at www.mirc.com
doesnt contain any backdoors/trojans/virii. You could have gotten any of those things thru other means of course (downloading a script/addon/mostanything). Its also possible AVP is giving you a false positive. With the info you gave, hard to give you a more specific ans as we dont know what it found or exactly where in mIRC. Guess i would start by running a diff virus scan (there are good online ones, like at trendmicro) to see what they find, checking remotes, check with kaspersky to see if they have had any false positives reported with mIRC.
hrm, seems to me there was something in the bug report area here last month about a false positive with kaspersky, use the search feature, set it to bug reports and expand the date range to 3 months with kaspersky as search term.