This is very different from virus to virus. Some malware, such as so-called ransomware software, is very banal; you may even have a ransom note directly on the screen. Other malware, on the other hand, does its damage in the background and can be very difficult to detect. It shows up, for example, through unfamiliar pop-ups or spam and the frequent crashes of your PC. The most common sign that malware is on your computer is that it is suddenly running very slowly.