These botnets (networks of compromised machines) were often used to relay spam. One type of spam, fraudulent emails that seek to dupe users into handing over sensitive account details to bogus websites (a practice known as phishing), grew markedly last year. In 2005, phishing represented an average of one in every 304 emails, compared to one in every 943 in 2004.
Another thing on the rise is phishing. Just about all of us have had to deal with scam e-mails or web pages before trying to lure information out of us, and as time goes on the scammers are getting better, making more realistic lures and using security flaws to their advantage more. It's likely that as a result of this, most anti-spyware and anti-virus companies will begin concentrating on eliminating phishing, which unfortunately for us is a much more crafty beast than the e-mail viruses of years past.