This is a symptom of virus in your PC. Recent viruses/worms keep sending emails from infected PCs without the user’s knowledge. If a machine is infected and is online, viruses use it to spread themselves through emails. They use their in-built SMTP engine and infected user’s address book for sending those emails. Immediately clean your PC with good and updated anti-virus software. If you don’t know how to do this, contact your hardware/software vendor for the same.