What warns that a computer is infected and needs immediate attention?

Answer 1

There are multiple signs. The most common are computer is unusually slow, you get popups even when no program is open, you get unfamiliar popups even on trusted websites, you have high speed internet connection but your Internet is slow and etc.

Answer 2

Fake pages advertising anti-malware programs often tell you that your computer is infected when it is not and that it needs immediate attention. Often, it is to get you to buy something, or to get you to install something. Then once you install it, you likely do have malware at that point. Some of the fake antivirus cleaners will clean everything but what they put in your system.