Its quite easy to send an email and just put some one else's email as the return or sender.
I had it happen to me and I was getting emails from people replying to it!
Have you replied to any spam emails?
Replying to a spam email is bad as they can see your email is live.
I suggest you run a virus scan and make sure you have up to date virus definitions - Basically make sure the program runs the online update.
also look for Sophos Anti-Rootkit and run that.
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