So, out of curiosity, I check out the mIRC forums. Turns out it happened to a couple of other people also. One person emailed Computer Associates, the antivirus vendor that was flagging it as a trojan, and got a reply. In this latest update, the software was having "mirc.exe" show up as a "false positive", meaning, somebody at CA goofed in the latest virus definitions. As suggested, I added "mirc.exe" to the programs "Exclusion" list, then re-downloaded and re-installed mIRC.
Needless to say, it's a pain in the tookus when something like this happens...