How to fix Hard Drive Critical Error?

My dell laptop started acting up a few days ago. The background went completely black and all of my shortcuts on the desktop disappeared except for outlook. The computer keeps restarting on its own and a spyware device that I did not download appeared on my screen and did its own scan. From what I've googled, this is not an actual error but a virus, but I do not know how to get it off my computer. All of my files appear to still be on the computer if I search for them, but do not appear in their designated folders. (I.e. my documents is blank, but I can search for a specific word document and find it). Any idea how to fix this and more importantly, how to fix this without jeopardizing my files?