The past two days I've received the following window(s) opening up on my computer screen:

First, a window opens in the middle of what I'm doing saying:
"Microsoft Antivirus has found critical process activity on your pc. You need to clean your computer to prevent the system breakage."

Quickly following that is a message from Norton Antivirus:
"High Severity"
"An intrusion attempt by 3dcdgfer.prowebprotect.nl was blocked"

I tried Googling what Norton stopped, but I couldn't find any information. The only thing that I did in common today and yesterday (as far as I can remember) is go to Epicurious.com, a site that I've used for years safely.

I'm running Malwarebytes Pro right now.

I think I'll stay away from Epicurious for a while and see if the messages stop.

What did you do immediately after you saw the message?
According to this, it's a message from a rogue antivirus that is trying to infect you, and the best thing to do is close your browser and the malware popup using Control-Alt-Delete
***Microsoft Answers forum***

Clicking on an "X" in the corner of the fake warning may actually install it instead of dismissing it.
The thing to Google is not prowebprotech.nl but
"Microsoft Antivirus has found critical process activity on your pc. You need to clean your computer to prevent the system breakage."

The problem may or may not be connected to epicurious.com
Even supposedly "reliable" websites can have infected ads.
Or the unwanted popup may appear long after you visited the website that was actually infected.

Be sure to delete everything in your browser cache. If the malware is being stored in there, it will keep reappearing.

Jenny,
I think I used both of the ways to get rid of the box, once with the "x" and another time with the control-alt-delete process; I don't remember which I used yesterday and which I used today.

Always use the Control-Alt-Delete to stop suspicious popups.
What's made to look like an "X" may actually be an "install" button.

"Always use the Control-Alt-Delete to stop suspicious popups.
What's made to look like an "X" may actually be an "install" button."

Thanks Jenny, I'll keep that in mind.

Here is a way to make a shortcut that you can pin to the taskbar to kill Internet Explorer and Google Chrome without using Cntl Alt Delete
All you need to do is copy the following text and then go to your desktop and right click and choose new then choose shortcut. Just past the text you copied into the text box on the create shortcut screen and click next. You can name the shortcut whatever you want and you can choose an icon if you want. After you create the shortcut you can drag it to the taskbar so you can click it with the browser window open. I have used these short cuts for years and they work good. Note not for windows XP only windows vista windows 7 and windows 8.

For Internet Explorer C:\Windows\System32\taskkill.exe /F /IM iexplore.exe

For Google Chrome C:\Windows\System32\taskkill.exe /F /IM chrome.exe

Thanks, Donald, that works great!