Monitoring firms are noting a huge spike in spam levels, beginning in October. MessageLabs, for one, suggests that spam-mongers are beefing up their Trojan-spewing botnets in preparation for the busy holiday season.

Another rising problem: image spam. Spammers have figured out a number of ways to embed text into images -- where anti-spam solutions based on text analyses can't get to it. And as security vendors have begun building image spam solutions, spammers have created a series of random images for separate mailings that, again, simply won't be flagged. Or they can put the text into multiple images in each mail. Or use obscure file formats.

Statistics from various sources put image spam anywhere from 21 percent to 35 percent of all spam sent.

Not only do the images elude anti-spam software, their malicious payloads are more likely to be launched by users. And they're bandwidth- and storage-eaters; the average size of an image spam is 18 KB, compared to 5.5 KB for a text spam.