a spammer can use your email address as the sender without using your account. Its a fairly simple hack of the header, and many mail systems dont check the sender.

Learn to set the filters on your inbox to split up incoming mail. Web based Email like yahoo may be doing that already, Outlook express and other pc based mail readers you have to do yourself. I send mesages to the admin of the emails that bounce to see if they log IP addresses.Your email provider should be able to tell if their system sent the mail.

If it is coming from your account , change the password, and scan your PC for virus. Download the free Kaspersky (?) or similar scanner.

I have filters for viagra, enlargement, Nigeria, JAL777 (I think - its an airline crash spam letter)  to a spam folder, renderosity, daz etc all go to a newsletters folder.