Applications / Windows Keep Getting Unselected

For example, if I'm typing in Word, randomly the Word application/window will get unselected, as if I clicked something else. I won't be able to type anymore until I click the Word window again, it's pretty annoying too since I'm a fast typer. And this happens with everything. Games, browsers, etc. I've also noticed when I have something fullscreened, like a video or a game, after a few minutes the start bar at the bottom of my screen pops up, and I'm assuming whatever was fullscreened got "unselected," and that's why the menu bar pops up. I have McAfee (I'm about ready to uninstall it), and Malwarebytes' Anti-Malware, and I've checked my computer many times but nothing has come up. This has been happening for over a month and I've just about had it.