Be very wary of anything that pops up on your computer, whether it’s an ad or a warning message. Often the most obvious response (clicking “X” or “OK” or “close”) is exactly what allows your computer to become infected with a virus. Use a popup blocker (although these are not 100% effective). If you do have an unexpected popup, right-click on the window and force it to close. If that doesn’t work, open your Task Manager (right click on the task bar, select Task Manager), then under Applications select the offending popup and click End Task. That should keep you pretty safe.