A Dos virus is a big threat for those who do not have good defence installed on the computer. Dos viruses are very risky malwares designed to search and delete *.COM and *.EXE files on the hard drive, or corrupt necessary system files by writing invalid data. The loss of these files may greatly slow down the computer, or make it run crash/freeze up randomly. If you keep on running your infected system, it will become more and more unstable, leaving your privacy unprotected.
A constructor, or virus constructor, is a malicious program attempted to creat new viruses on the computer without having any programming skills, as it has an interface that allows to automatically decide the characteristics of the generated malware such as type, payload, target files, polymorphism, etc. Constructor may seriously break into a system by making use of the fragile programs or via infected application downloads and then gather and send your important information to a remote controller.