But sometimes you have no choice. Scan every file on the disk, not just the program files. Do this even for shrink-wrapped software. Likewise, when you give a floppy to somebody else, always write-protect it. CD-ROMs are less risky, but scan them the first time you use them anyway.
Many computer users believe that downloaded files are the most common source of viruses.
To be safe, download all files into a special folder on your hard drive. Then be sure to scan those files before you open them. Some email programs will automatically open certain attachments with the appropriate program. This is a nice touch and can make reading attachments a bit quicker, but it's also a potential virus nightmare. Disable this function in your email program, then scan any attachments you receive before you open them.