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. If the need to do so is there always save the file to your hard drive before doing so. Delete chain emails and junk email. Do not forward or reply to any to them.