These emails are effective because they look genuine, but the attachment is a virus which can gain access to sensitive data. If you are suspicious of an email or attachment, the best thing to do is to contact the source directly by telephone asking for confirmation that it is a legitimate email, if it is not, delete and report it immediately.
If you have already clicked the attachment, do not despair. Immediately turn off your computer to stop it spreading to other contacts in your address book. You should then contact your IT support team or talk to us.