There's so many anti-virus tools that are currently available, i personally use windows devender and Avast.
I want to tell you a little bit about my experience with computer viruses,
Knowing for sure the computer is infected with a virus or not other than the results of anti-virus detection, based on my experience, it will be very difficult. Many millennial viruses or malware can disguise themselves as "normal processes" on the operating system (especially Microsoft Windows). Trust the results of anti-virus detection if you suspect the computer is infected.
If you look a computer that is slow does not mean being infected with a virus. It could be that computer resources are not sufficient for today's software needs.
The most effective way I always use, if the computer is infected is:
Scan with the anti-virus that uses the Live OS system.
Create non-administrator users to use computers. Viruses, especially those that live on Windows, require Admin access to change the system.
Always backup important data either offline or online. Google provides 15GB for free that we can use.
If the anti-virus does not successfully clean the virus or I am hesitant, I will reinstall the OS and do the second method above.
Start not doing software piracy because it is one of the main gateways to spread the virus.
My recommendations are Kaspersky, Norton, ESET.
Avoid using free antivirus like AVG, because the detection method is quite bad, so if there are virus on your PC, there's a high chance AVG won't be able to detect it.