You should, of course, be wary of any file attachments a stranger sends you. At the least, you should check such file attachments for viruses before running them. Use a virus checker regularly. Freeware, shareware, and commercial anti-virus programs are widely available. Which program you use isn't as important as how you use it.
Most people get into trouble because they never bother to check their computer for viruses.
Most viruses spread through floppy disks, so isolating yourself from online services and the Internet will not protect you from viruses. In fact, you're probably safer if you're online, simply because you'll have access to anti-viral software and information. Not necessarily in order of importance .