In 90’s there were many computer viruses which usually forced users to reinstall their OS. Although, those virus programs seems to be kiddy now, but it used to be a big problem. The Malware Museum now allows you check those computer viruses without harming your computer.
The Malware Museum is the collection of interesting viruses which affected the MS DOS os back in 80’s and 90’s. There was no antivirus and reinstallation of OS used to be the only option.
You can see the list of various MS DOS based virus here and check what kind of infection they did in past.