Why is my computer going crazy?

All of a sudden, my pc has these issues: ads on internet explorer keep popping up on my screen for several seconds – even when I’m not actively online, my “game launcher has encountered a problem and needs to close” – multiple times, things are slow, and it’s just being generally uncooperative. Mind you, I did have a lapse in my security software (CA Anti-Virus), and I renewed it today. Ran a scan & removed/quarantined problems, but it’s all still happening. Why does it hate me so?