888-820-0988

This number appears to be a scam. It keeps showing up, stating a virus has invaded my computer, however; Webroot finds nothing when it scans. I have not called the number.

The number keeps popping up on my computer-- wants me to call it for help with my virus. I do not have a virus.

If it is showing up while you have your web browser open, it is most likely just a scammy web advertisement for the fake AV product.
Just exit the web browser or use the Task Manager of your computer's operating system to close the browser and avoid the website that the popup was on.

If it is popping up at random when you are not using your web browser, then you have an active scare-ware trojan running on your computer.
For this, either have a retail computer repair service, like Best Buy Geek Squad or such, or a trusted computer guru friend, normally among your circle of friends someone known someone who is very good with computers, clean your computer.
Or you can use a trust worthy malware and anti-virus scanner and cleaner to fix it yourself.
The software I generally use to clean computers in MalwareBytes:
www.malwarebytes.com