After you got a new computer, usually it will come with Anitvirus software free trail. However, after the trail period expire, you will suffer from pop ups asking you to register and pay. I find AVG has free version which play really nice with Windows. No pops up, completely free and get the job done. Please make sure you uninstall the old AntiVirus software before you install.
Go from this Link and click "Free basic Protection"