One of the worst things that one can do when experiencing computer trouble is rushing to attempt to fix a problem without really understanding what is happening. There are many types of malware, and some of the more insidious ones disguise themselves as legit or even antivirus programs. Downloading the wrong program in an attempt to fix your computer will only make things worse. Here is how some of these programs work.
If your computer is already infected with malware, sometimes you can get popups which prompt you to install more files, or potentially pay to upgrade a program. Don’t listen! If you get a random popup warning of viruses telling you to pay money to upgrade your antivirus, it is very very likely that following these instructions will at least further infect your machine, and potentially even give your credit card number over to scammers. These programs use the foothold that they have created to masquerade as a legitimate program. When users with little experience suddenly receive instructions telling them how to fix their problems, they often will initially follow the instructions, trusting them implicitly. Often it is not till later that the users realize that perhaps they should have been less trusting. If you have followed any steps such as this, there is a very real chance that your computer is completely compromised. If this is the case, all malware will need to be removed by a professional, and steps may be needed in order to mitigate the loss of private information and credentials.
Other types of malware which masquerade as antivirus programs do not need to have a foothold installed. These types of malware rely on an official looking website or other phishing tools in order to get a desperate user to install their software onto a machine. In this case, a user might be simply using their search engine of choice to look for an antivirus program to install on a potentially infected machine. If they stumble on the website for a program which appears to be or pretends to be malware but is at best useless and at worst malicious, they might install and run the malware before realizing that anything is wrong. As above, if this has happened, there is the potential that this and all connected systems have been compromised. The only thing to do in such a situation is to seek professional help. Never install an antivirus if you are even slightly unsure of its credentials.