How far did computer viruses ever got to you?

I resisted getting a computer because I heard so many bad things and had a lot of friends whose time seemed to be consumed by them. I finally gave in because my wife needed it for work. But I didn't think we needed security because we weren't going to put a lot of personal information on it. Well it didn't take long to find out how little I knew. Our computer was taken over. A friend got it off but it had corrupted so many files that we had to reformat. Nothing worked right. After installing security and having reformatted she was doing research to do with hospital sterilization and called me to the computer. She said she went to a sterilization sight and something popped up but security popped up with it and said it had blocked something. Later I went to history to see what she was talking about and it was a trojan that was blocked. So these can come from anywhere.