My NAV failed to protect my system and instead became incompatible with Webroot Secure Anywhere while allowing the virus to work in my system

Today, my student inadvertently plugged in an infected thumb drive into my system and messed it up despite the fact that I have NAV. My NAV instead of protecting my system, allowed the virus in and rather started indicating that I should uninstall the Webroot Secure Anywhere. Less than three weeks ago when I renewed my subscription the NAV became incompatible with the Webroot Secure Anywhere but after three days, it started working again. I have tried scanning my system with Norton Power Eraser but it did not help. I wonder if anybody has an advice on what to do in this situation? Is there any other AV that can help me? Thank you in advance.