Remember to keep your anti-virus software up to date. Your anti-virus software will only help so much, though. New viruses are always coming out, and you may get a virus so new that the software does not yet know how to detect or remove it. Do not open any files attached to an email from an unknown, suspicious or untrustworthy source. Do not open any files attached to an email unless you know what it is, even if it appears to come from a dear friend or someone you know.
Some viruses can replicate themselves and spread through email.
Better be safe than sorry and confirm that they really sent it. Do not open any files attached to an email if the subject line is questionable or unexpected. Update your anti-virus software regularly. Over 200 viruses are discovered each month, so you'll want to be protected. Do not download any files from strangers.