Years ago, you knew when you had a virus, it was quite obvious. It normally told you, it would take over your screen stopping you from working and the race was on to try to remove it before it trashed all your files. There was no financial gain for the virus creator. Today most viruses are designed to go unnoticed, working secretly in the background. Some known as rootkits are practically invisible and you could have one on your computer for months and never know.
Also, these days you are dealing with viruses, not a single virus. You start off with a single infection, which can rapidly become a team of malware each with different tasks. Some are designed to protect the main virus, others to hijack resources and connect to other infected machines to form a network. The first thing they tend to do though, is make your antivirus software next to useless. Either way, getting rid of them can be extremely difficult and time consuming.
If your antivirus software identifies a virus on your computer while you are surfing the internet or reading an email, chances are it will deal with it and you can go on about your business as normal. If though, your antivirus software spots a virus already on your computer as part of a weekly scan, it's best to run a second scan from another antivirus firm. A second opinion so to speak. All you need to do is visit the website of a mainstream antivirus provider and run a FREE on-demand scan. I personally like Housecall on-demand scanner by Trend Micro and the Sophos virus removal tool. If the second scan is not clear, let the software remove the infection, then run the scan again. Remember to set any scans to be a ‘Full system’ rather than a ‘quick’ scan.
If you have suspicions that you may have a virus, chances are that your antivirus software will not spot it. Even if your antivirus software hasn't been sabotaged, it may still not spot it as they tend to be a week or two behind with the latest viruses. If you believe your computer is infected, locate another computer (preferably a tablet or smartphone) and change all your passwords for any internet-based services. You need to stop using the infected computer and try to run an on-demand scan or install another antivirus program. You may find that you cannot access antivirus websites, which is the virus defending itself. Download what you need from another computer onto a USB stick, I like Malwarebytes in these situations, together with the setup files for another internet browser that is not already installed on the compromised machine e.g. Chrome or Opera.
If you do not have access to another computer and using a Windows operating system, there is a basic web browser built into HTML Help that you could use to try and download Malwarebytes or another antivirus tool like the Sophos virus removal tool. To run it, open a command prompt (Windows key+R) then enter "hh h" (don't type the " " speech marks) and hit enter.
This will open a small HTML Help window with a ‘This page can’t be displayed message.’ This is normal, resize the window by dragging a corner, then click on the question mark in the top left corner to bring up the following menu.
Click on ‘Jump to URL…’
Next enter either www.malwarebytes.org or www.sophos.com/en-us/products/free-tools.aspx and click OK. Hopefully you’ll be able to download something to start dealing with the virus. Once again, you'll need a second opinion that the infection has been removed like or www.eset.com/us/online-scanner and even a third scan may be required depending on the complexity of the malware. From the scan results, note down the name of the malware and look it up on one of the various online virus encyclopaedias available from the antivirus providers. If a specific removal tool is available, download and run it, or follow the step by step removal instructions if there are any.
The biggest issue you have with dealing with viruses yourself is thinking you've removed the infection, but when you restart your computer, the virus is back. So, you can see why removing a virus can be so time consuming, which is why I recommend taking your computer to a large computer store that offers virus removal for a flat fee. It may be quicker in the long run and less stressful.
You can find further advice by the UK National Cyber Security Centre, called, ‘What to do if you (or your organisation) has been infected with malware:’ at:
https://www.ncsc.gov.uk/guidance/mitigating-malware#section_4
Lastly, sometimes the criminals use trickery to look like you have a virus using an animated webpage like this, so it looks like you have been hacked. So be wary of perfectly timed help.