As a global trend, the amount of spam floating around the net has increased by a significant degree the past few months. While you may have noticed it yourself, there are many who's inboxes are becoming more inundanted with junk email, and some security companies are pointing the finger at botnets. As spam blacklists become larger and ISPs begin to filter on their own more, compromised networks of PCs are becoming increasingly more used as a tool to deploy spam mail to circumvent those measures:

While bulk emailers have, in the past, sent unwanted messages from a single server, increasingly the spam emanates from networks of compromised PCs, known as bot nets. The level of junk email has increased almost in lock step with the number of compromised systems used for spam, said David Hart, the administrator for Total Quality Management.
"What is most alarming is that new clients - internet addresses that we have never seen before and which could be new infections - have tripled since June," said Hart, who posted a chart tracking the growth on his Web site this week.

While the amount of security offered on any particular home and work desktop is more than it ever has been, there are more PCs than ever online. The article even links to a graph showing a sudden growth in the amount of spam. Likely, there is a larger reason behind it, such as an unpatched exploit common among many machines or previous infections lying dormant.