Monitor your computer for unusual behavior.
Your computer may be infected with malware if it:
slows down, crashes, or displays repeated error messages
won't shut down or restart
serves a barrage of pop-ups
serves inappropriate ads or ads that interfere with page content
won’t let you remove unwanted software
injects ads in places you typically wouldn’t see them, such as government websites
displays web pages you didn't intend to visit, or sends emails you didn't write
Other warning signs of malware include:
new and unexpected toolbars or icons in your browser or on your desktop
unexpected changes in your browser, like using a new default search engine or displaying new tabs you didn’t open
a sudden or repeated change in your computer's internet home page
a laptop battery that drains more quickly than it should
source: https://www.consumer.ftc.gov/articles/0011-malware