This is the 9981st most frequent Russian word.
антивирусом
"Антивирусом" means "antivirus" (software that protects against computer viruses).
The word 'антивирусом' is used here as an object, indicating the software being installed.
Я установил антивирусом на свой компьютер.
I installed antivirus software on my computer.
In this instance, 'антивирусом' refers to the software being handled or used in action.
Работа с антивирусом помогает предотвращать угрозы.
Working with antivirus software helps prevent threats.
Here, 'антивирусом' functions instrumentally, describing the tool used for protection.