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double extensionA way to trick users into opening a virus. Many people have learned that text files (.TXT) and image files (.GIF, .JPG, etc.) are safe to launch because they are data and not executable software. They have learned to be leery of .EXE, .VBS and other extensions that are executed immediately. Thus, virus writers try to trick more people using double extensions, so "I LOVE YOU.TXT.vbs" is really not a .TXT file, but a .vbs file, a Visual Basic Script that is executed immediately.
You Need to See the Extension
Some mail programs may actually remove the .vbs at the end of the name, leaving users completely helpless to make a determination even if they knew what to look for. This mentality is pervasive. Out of the box, Windows defaults to hiding file extensions in all displays, which is beyond absurdity considering the importance of this file identifier in everyday operations. See dangerous extensions and Love bug.