I suspect the first thing to do is change your password and make it a strong password and see if it makes a difference
You say you do not send any emails
--- Does that mean you really do not send any emails at all or you are not sending out the ~50 emails a day that you mention?
--- Do you have an address book and are some of the email addresses included in the list of emails that are being sent out
--- In other words, there might be a total of maybe 55 messages which in this case there are 5 email addresses taken from your address book!
--- The reason I'm asking is because I'm trying to figure out whether or not your address book has been hijacked and/or the spambot is just using your account for their own purposes
I think it would be good for us to know who is your mail host?
--- Did your mail host make any recommendations on how you can thwart the spambot in addition to ensuring whether or not your computer is infected?
--- Do you have filtering capabilities so that you can manage which emails are allowed to even come in?