All it takes to get on basically all spam lists is posting your email addy - say here - and then opening one.
The spammers have bots looking through forums and newsgroups for active email addresses. Then they include a picture in their email which is requested with a code like
http://forums.speedguide.net/image.php?u=13331&dateline=1035824647
which actually is my avatar. But, as the "?" signifies in the address, the script producing the image is given a code. This code can be traced back to your email, and hence they know that you actually opened the email. Your account is definately active, and hence your address is a prime spam target. Because of this I would definately either turn off the automatic preview pane in outlook (express), because it automatically opens the mail and requests the images. Or, you tell your firewall to allow Outlook to access nothing but your mail servers. Aside from viruses, this is enough of a reason to turn it off.
Extra credit: if you're unsure whether an email is really spam or not, you can simply find out safely in Outlook Express by right-clicking the mail, select properties, go to the "details" tab and click the "message source" button. Then you can read just the text of the email, without any danger of activating code found within or download said pictures to mark your account as active. Almost all legit emails have sufficient normal text in them to clearly identify them as safe to open.
So if you're about to post your email addy, like say for example it were furious@fords.com, you'd put it as furious<at>fords com, furiousNOSPAM@fords.com, you know, not having your actual email address identifyable by those searchbots. Most forums, like the VBulletin that SG is using, allow you to send private messages to give your email out and/or have IE forms for sending mails to members without actually seeing them.
I've had my main email account for 1 1/2 years now, I've gotten only one or two spam mails on it. Just be careful who you give it out to and how.