First determine if the messages are being sent from your computer. You might
need to get a copy of one of the messages from someone that received it.
Then you need to know how to look at the headers. Then you need to know how
to determine if it was your computer that sent the message. If the messages
are being sent by your computer, then you need to get better software that
can detect and remove the guilty malware.
If it is not sent by computer, then you need to determine if the email is
actually being sent using your email address, or if the email address is
being spoofed. I think there is a difference but I am not sure. Spoofing
means that the message headers lie about who sent the message. There is very
little that you can do to prevent your email address from being spoofed.
There is something called SPF records that might help but I can't help with
that. When I tried to learn about SPF records and I asked them some
questions, they just replied as if I need to read the documentation yet the
documentation is (was) nearly useless. For everything about SPF records see:
http://www.openspf.org