Free information on the web isn’t free; there are costs to provide it. Pop-ups: advertising, offers, and social media, are all part of the “cost” of free.
There is no such thing as perfect anti-virus software, so it’s wise to have another line of defense when online.
Advertisements popping up on a new computer bring several things to mind, including what many call “crapware.”
Usually a new window will open on top of previous windows. There are several things that may cause a window to “pop under” the window you’re looking at.