The new spam-generating programs create random subject lines and random text in the message body in order to defeat Bayesion spam filters. (like
Spam Assassin) Bayesian filters 'score' an email based on words and phrases to arrive at a spam probability factor. For instance, the words "Viagra" or "beastiality" would have higher scores than "brakes" or "engine". This dilutes the overall score and allows the message to get thru. The spam generator programs create a multi-part MIME-encoded message, with the spam message ("Enlarge your p3n!s!") in the first part, and the garbage text in the 2nd part. Many times they bastardize the headers so your email reader doesn't even see the 2nd part. I got one the other day with the text %RAND% repeated something like 200 times!
Internet worms have moved beyond being simply annoying; now the worm writers are leasing them out to be distributed spam distribution network. The new worms also have expiration dates, which belies their economic intent. Basically, advertisers will be able to rent a worm in the way they're able to rent a phone or mailing list now.
All of these unsecured home PC's with a broadband connection are being used much more efectively than any open SMTP relay could ever be. At the very least, you should install something like
ZoneAlarm so you can control your inbound and outbound traffic.