And how can i stop it from doing it because people are getting annoyed.
It is either a virus that you got from opening an email or your account has been hacked. Run your virus protection on your computer and clear all the viruses and then change your email password incase you were hacked. Never open emails from people you dont know or click on links in emails from people without confirming its safe first.
You have a virus.
If you do not have anti-virus software get some now and use it.
You can get FREE software called AVG anti-virus software from CNET.com
A great site for SAFE software (much of it free) is
http://funadvice.com/r/3ju5v5fb1h which is a site that I have recommended for many years.
change your password because youve obviously been hacked. if it continues, you'll just have to get a new email address.