Well I can't help you much here but I know a way how you can stop it next time if something like that happened again.
When you are downloading stuff from the internet from websites and starting to seeing these weird messages that pop out from no where. Usually those message asking you to click on the message box; well do not. If you accidentally click on it or sometimes it does automatically without you clicking it (you'll see a quick loading bar); then press the ctrl and alt and delete the same time really quick, click Start Task Manager, next pick which task you want to end, then click on End Task button. It worked for me even though my ant-virus expired.
Tip: You have to do it really fast.