Prevention
An ounce of prevention is worth a pound of cure. You should be running virus protection and regular updates. Have virus eradication (different than protection) software already installed. An example is Malwarebyte Anti-Malware. There are also root-kit specialty virus removal.
Have backups of you data. Cycle them so you also have some old(er) backups. Don't leave your backup device plugged in - if the virus is going to corrupt or lock data then it has access to your backup. Cloud service for the $5 / month is money well spent.
Use Firewall protection.
Symptoms
Sluggish. High CPU but no programs admits to using the CPU. Update on OS and / or virus protection fails. Virus protection won't start.
Removal
Removal is not always successful and it can be very time time consuming but if it works then you still have all your programs, setting, data.
I have gotten dozens of viruses and have always been able to remove them. In one case it had hacked up the registry enough to be a problem. But I was going to upgrade anyway so I just applied and upgrade.
Hopefully you already have removal program(s) installed.
Don't just Google Virus Removal and download the first you find. Some are just viruses themselves. There are known names. And some good free stuff.
Disconnect from the Internet. A virus will typically disable virus protection so 1 virus can quickly turn into 20. And it may be scanning to PC to send data to the mother ship.
Run you virus removal program(s). Sometimes you need to boot in Safe Mode. By booting in Safe Mode some of the viruses don't load so they are easier to find and delete. Hopefully that cleans up some stuff.
Connect to the Internet and update the virus program and run them again. If they say clean you may be good to go.
The run all your OS updates.
Some times the virus is gone but it hacked with the registry and thing still don't run right. There are registry repair tools - typically free from the OS vendor.
Recovery
Run recovery from you recovery partition or original media. Make sure and immediately install updates. You might lose minor stuff with a recovery.
Worse Case
Some viruses require a reformat and re-install. The problem here is you have to re-install EVERYTHING.