Help! I am a secret spammer

[Chuchinchow]

I woke up this morning to find my in-box full of irate messages from some of my regular correspondents, complaining that I had sent four consecutive spam messages to them.

I am more than surprised, because I have the following prophylactic software programs installed on my computer:

AVG - paid and totally up to date
Ad-Aware - ditto
CCleaner - up to date
Malwarebytes' Anti-Malware - paid and totally up to date
RegCure - ditto
Spybot - totally up to date

Would some kind soul please tell me where I am going wrong and what more I could do to prevent any further unwanted transmissions of spam?

Thanks in advance.