Are you sure the emails are going from your account? How do you know?

If you know there is a problem because your friends are complaining, it could simply be that your address has been harvested from someone else's PC, or from an email you may have ill-advisedly replied to once (one of those viral messages that do the rounds from time to time would do it) and the emails now being sent have spoofed headers which make it look like they have come from you, when in fact they haven't...