annoying attachment in my e-mails

My problem is that every time i send an e-mail (with HMTL format) an annoying attachment is sent with it without my permission. I use the microsoft outlook 2003 and I am really upset with this story as people calling me asking what is going on. It’s not an attachment that I have choose or want to send. I can see it in my Sent items and it is in fact a gif file containing a bullet. Pls advise…..