If your computer is behaving strangely, running slowly, or you have clicked something suspicious, a virus or malware infection is possible. Follow these steps carefully and in order. Do not skip ahead.
If you suspect an active infection, disconnect the computer from Wi-Fi or unplug the ethernet cable first. Some malware transmits data or contacts command servers. Cutting the connection limits what it can do while you deal with it.
If there is keylogging software on the machine, logging in to your bank or email from it hands those credentials to the attacker. Use a different device for anything sensitive until the computer is clean.
Malwarebytes Free is a reputable second-opinion scanner that catches things Windows Security may miss. Download it from malwarebytes.com, install it, and run a full scan. Remove everything it finds.
After the scans come back clean, change the passwords for any accounts you accessed on this computer in the past few months. Do this from a different device or from the same computer only after verifying the malware is removed.
If scans continue to find malware after removal, if the computer behaves erratically, if security software is disabled and will not re-enable, or if you believe sensitive data may have been accessed, bring the computer in for a professional cleaning. Some infections are too deep to remove cleanly without specialized tools.