I recently updated Mirc and since then, I am unable to connect to Undernet.

I keep getting the following message:- AUTO [1] (2.103.54.44) Infected with a virus or trojan, please clean your system. (P321).

I have scanned my PC and no issues with virus etc are reported. Anyone any ideas?

it's possible they either banned a range of ip addresses which snagged you, or else you recently received an ip address used by someone who had been affected. Given the odds of someone else also going to undernet, I'm guessing the former is more likely.

Back when I had a floating WAN ip, I'd occasionally see this kind of error. I googled the text of your message between the P321 and the IP, and got hits from an undernet forum that doesn't look like it's had posts for 5 years, so not sure if their mention of emailing an abuse address will find an inbox read by a real person.