What happens when a computer gets a virus?

You will start to get ad pop-ups like crazy, and your computer will be a lot slower. I'd recommend that you be careful downloading things, make sure it's the right thing you're downloading. Some people make false websites. Also the best thing to do is get a usb drive and store everything you want to keep and reset your computer to factory in your settings.

It basically starts acting strange and it slows down 

Idon't have a single clue tbfh

You forgot to add the possibilities. we don't know the options from which to choose.