Annoying big, colorful ad popups here, too

[cmiyc]

I'm running Vivaldi 4.1.2369.16. Suddenly I'm getting big, flashy, urgent ads and fake-scary notices about viruses and so forth in the lower right-hand corner. I notice that this has been a problem since 2018 or so, but it just started with me yesterday. Our computers are heavily dug in: IObit Advanced System Care, IObit Malware Fighter, IObit anti-ransomware engine, Bitdefender engine, HitmanPro Alert, and ExpressVPN (which I admit is usually turned off, because we have it mainly for department stores, hotels and other places with unsecured networks). I use the Startpage search page. I usually neglect to use private browsing because I just forget. Can someone direct me to the latest developments and recommendations concerning this? What I get with a web search is scattered over the last 3 years.

[TbGbe]

@cmiyc
Maybe some website(s) have a background "Service Worker" installed.
Check/Remove them with:

Open internal Vivaldi page vivaldi://serviceworker-internals
Hit Unregister for all shown entries
Restart Vivaldi

Also could be caused by "bad" extensions.
Check for this using Guest View which is a profile with no extensions installed.

[Catweazle]

@cmiyc , IObit itself is suspicious. It isn't a problem of Vivaldi. If you use Windows 10 you don't need any third party security or optimizing tools, all the IOBit crap and the other AV are redundant and apart of this , running several AV isn't a good idea, because all of them scan the same system files and can cause that one blocks each other. Adding, that Windows 10 has a Sandbox system which avoid that third party apps can accede the kernel.
Apart from the VPN, desinstall all this crap and activate and update the Windows Defender (which is more than enough, include against ransom ware) and make a complete scan of your disk. Aditional you can download the AdwCleaner, or better the Panda Cloud Cleaner (they only work on demand) to eliminate all the adware you has caught since 2018, if the Defender has not already done so (very likely).
With this and the ExpressVPN (the only good app of what you named) you are well protected.

Iobit has a bad habit of making different changes that make it difficult to completely remove it from the system, it can even make the system unusable if the steps described below are not followed.

From Microsoft support about IObit

Hello ******, welcome to the Microsoft community, my name is Bruno Leonel, an independent consultant and I would be happy to help you.

Advance System Care is technically not a virus. It is more appropriate to be called a PUP (potentially unwanted program). Its nature can be as aggressive as a computer virus, but it is never replicated after it is installed on the PC.

Step 1: Initially, press the Ctrl + Shift + Esc keys to open the Task Manager. In the Details tab, find the line diskdefrag.exe, click on it with the right mouse button and then click End Task

Step 2: Uninstall Advance System Care from Windows
1 In the Windows Start menu, type Uninstall in the Search field. Select Apps and features from the list.
In the next window, look for the "Advance System Care" item in the list and then click on Uninstall button.
When asked for confirmation, click Uninstall to start deleting Advance System Care from the Windows operating system.

Step 3: Remove the remaining items from Advance System Care with Malwarebytes (PD I think it is better with the Panda Cloud Cleaner)
Click the Scan Now button on the scanner console to ensure that it completely checks the PC for any presence of Advance System Care and other forms of threats.
Malwarebytes Anti-Malware will issue a list of identified threats. Check all threats and remove them from the computer.

Step 4 Uninstall secondary programs
First, open the Control Panel and go to Programs and Features. In the list of software installed on the computer that appears, check for the IObit Uninstaller and Surfing Protection programs. They are offered at the time of installing Advanced SystemCare. If they do, uninstall both before proceeding to the next steps.

Step 5: If you have browser problems
How to delete changes made by Advanced SystemCare in the browser
Advanced SystemCare makes some changes to your browser settings that can be manually reversed after uninstallation. First, open the browser extensions page and disable or remove the Start Page 2.0 plugin. Then, delete cookies and browser cache.

I hope this helps, feel free to post again. We will be ready to help!