This happened to me just a short while ago. First thing I knew about it was, I could not access the internet and when I rung the helpline, my account had been suspended because my computer was banging out spam from my email account and users had complained about it. Next day I also received a letter from my internet provider.
I was recomended to do a full anti virus scan and I found about 5 trojans which I deleted, rung the helpline again, who reconnected me to the net and advised me to install "Malwarebytes" ( a free spyware, trojans program etc) and scan my system with that which found a couple more trojans on my system which I deleted which AVG antivirus, Adaware and Spybot on my system did not find