Ihave been recieving mail from my contacts reporting that they have been recieving bogus e-mail from my address that I never sent. How do I go about stopping this?1 person needs an answerI do too

Ihave been recieving mail from my contacts reporting that they have been recieving bogus e-mail from my address that I never sent. How do I go about stopping this?http://www.google.co.uk/search?q=email+hijacked&rls=com.microsoft:en-gb:IE-SearchBox&ie=UTF-8&oe=UTF-8&sourceid=ie7&rlz=1I7GGLT_en&redir_esc=&ei=khnwTNWiGZG4hAfw9L3RAw