And what about error scan and defragmentation programs. Currently, viruses are the biggest threat to users. That's the good news. The bad news is that the current crop of anti-virus products is inherently flawed. The best way to avoid a virus is to not put yourself in danger of becoming infected in the first place.
Some practices are a lot riskier than others.
This is a prime channel for hacker to distribute new viruses. Don't open email attachments from strangers. Don't open email attachments that you don't recognize, even if they're from someone you know. If in doubt, contact the sender before opening the attachment.