Weird popup, computer running slowly?

My computer is a lenovo yoga ideapad. It's on windows 8. Lately, I've been getting this weird popup that disappears before i can even read it. All I've ever made out is "sandbox" something. Ever since that started happening, my computer runs slow and gets hot really quickly. My antivirus isn't installing, so I've tried using windows defender and running a scan to find any malicious viruses or anything, but every time I do, it stops halfway through and says that windows defender has been turned off? Please help! I don't have the money to go get it fixed, I'm only 15. Any suggestions help-I'm also not very good with technical aspects so simplest way you could put it would be nice.

Do a factory reset before it fries itself!
Hold down the shift key and restart it keep the shift key down until you see trouble shooting choose that option and factory reset it don't fool around doing a refresh. Reset it before chips turn to toast and you have to buy stuff