Why not instead of a subscription service, have a punishment fee. So people can have your software forever and if they get very few virus infections, then you charge them nothing. But if all of a sudden they start increasing the number of infections, you start charging more and more money depending on the infections rate of that computer. This way the more vulnerable the user makes it for themselves, the more they get charged. Or, the more they do stupid stuff, the more money they have to pay to fix it.
This way you have Norton on your computer all the time and you aren't kicked off after the trial period and that keeps the software running.
Also, here is a way to save money.
When you search for Norton, don't have it so you pay for the Google Adwords for the Norton search on Google. Because if I search for Norton Anti-Virus, you are already the top search right there and the user is looking for that term. So it makes no sense to pay for that click. You already have that click and people might be just clicking that ad to get to your site and you pay for each one of those clicks. They were already going to your site and you are paying for them to already go. You shouldn't have to pay for that. Because look at how much you are paying for your own search term and compare that to how many clicks you get with other search terms. They were already coming and you are now paying for that.
They might argue that it gives a chance for competitors to advertise on your search term, but really... That's the concern. How much are you paying for that to block them. Is it worth it? The user has already committed to finding you. Let other ads come up around your search term. Competitors would be willing to pay for that search term, but it makes no sense for you to pay for it.
Just thought I'd help.
Regards,
George