Do I have a virus when virus warnings pop up?

Almost never. The boxes come from website notifications you once clicked Allow on, and they can display any text at all, including invented virus warnings. No web page can see what is on your computer, and a real scan is only ever run by Windows Security on the machine itself.

Can Microsoft or my bank warn me in a pop-up?

No. Microsoft, Apple and Swedish banks never send warnings as pop-ups and never put phone numbers in them. A box asking you to call a number or download a program is always an attempt at fraud. Close the tab and never call the number.

Will resetting Chrome delete my bookmarks?

No. The reset restores your start page and search engine and switches extensions off, but bookmarks and saved passwords stay. You can turn back on the extensions you chose yourself afterwards.

Pop-up ads and alerts in Chrome? How to switch them off

Boxes appearing in the corner of the screen with adverts, prizes or claimed virus warnings are almost always website notifications. At some point you clicked Allow on a site's request to send you notifications, and it now sends whatever it likes. It is not a virus, and it takes a few minutes to switch off for good.

No web page can see what is on your computer. A box claiming your machine is infected knows nothing about your machine.

Try this first

Open Chrome and click the three dots at the top right. Choose Settings.
Go to Privacy and security, then Site settings, then Notifications.
Look under Allowed to send notifications. Every site you have approved is listed here. Most you will recognise. The ones you do not are the ones sending the boxes.
Click the three dots beside each site you do not want to hear from and choose Remove. Remove all of them if you are unsure, since you can always say yes again to a site whose notifications you actually want.
Choose Use quieter messaging at the top of the same page, so websites stop asking in future.
Restart Chrome and use the computer for a while. Usually this is where it ends.

On a phone: in Chrome on Android, tap the three dots, Settings, Notifications, and switch them off site by site. You can also hold your finger on a notification as it arrives and turn off that sender there and then.

If that did not help

Look through your extensions. The three dots, Extensions, Manage extensions. Remove anything you did not install yourself and do not recognise. Pay particular attention to names containing coupon, search, shopping, PDF or converter.
Check the search engine. Settings, Search engine. If it shows something other than what you chose, set it back to Google or whichever you prefer.
Check the start page. Settings, On startup. If an address you do not recognise is listed, remove it.
Reset Chrome. Settings, Reset settings, Restore settings to their original defaults. That puts back your start page and search engine and switches off every extension. Bookmarks and saved passwords are not deleted.
On Windows, scan the computer. Type Windows Security in the search box next to Start, open it, choose Virus and threat protection and click Quick scan. It is built into Windows and costs nothing.
Check the program list. Settings, Apps, Installed apps. Sort by install date and see whether anything appeared the same day the boxes started. Uninstall what you recognise as unwanted.

What never to do

Do not download a program a pop-up recommends, however convincing the box looks. Never pay for a cleaner or optimiser that appears in an advert. Never call a phone number shown in a warning box, whoever's name or logo is on it.

Microsoft, Apple and Swedish banks never call you about your computer, and they never display phone numbers in a box on a web page. They also never ask you to install remote-control software or to read out a code. If you get a call claiming to be from any of them, hang up and call back on the number printed on the bank's own website or on your card.

If the boxes return after the reset, if the start page changes itself again, or if the same thing happens across several different browsers, something is installed on the computer. That is the point to ask a person rather than keep clearing.