Maybe the add on changes something in that exe file and your virus stuff dont like it, saying it is infected based on the database in your virus proggie, the two arent alike and the norman virus thing doesnt agree with it. Ive never been hit with any virus, ever. But I do have Norton virus and Norton utilities on my system. (knock on wood)
Remember that one virus that got sent thru tons of emails and it crashed alot of corporate systems? I saw that in my email and imediately deleted it. Since then, if I see something in my inbox I dont recognize, it gets dumped without question! I have my Norton active to scan downloads as they are downloaded, so if it beeps a warning, I can cancel the download without worrying afterwards. Then it will scan and automatically purge any files or zips with viruses. The key of course is to stay up to date with the anti-viruses database and upgrade your proggie often. The best thing to do is not accept things from sources you dont know and stay away from certian sites that like to transmit these viruses.