Executable files that contain instructions for a computer to perform an action, COM (. Com) files are for DOS-based systems and tend to run faster than EXE (. Exe) programs. Viruses often infect COM files. When the COM file executes, the virus executes as well, often loading it into memory. The Microsoft Windows operating system treats files with a COM extension the same way it treats other executable file types. Some viruses and Trojans use a filename ending in COM (e.g., http://virus. Com). Typically, these portable executable files are not real COM files.