So I just finished deleting over 100 spam emails from my Outlook inbox that came in since this afternoon, emails that normally should have gone straight to my Junk folder. For some reason, Outlook wasn't filtering the junk. Various disaster scenarios ran through my mind (virus? trojan? email worm?) but a bit of Google research revealed that installing various Office 2007 beta components (of which Microsoft Expression Web Designer is one) seem to disable junk mail filtering in Outlook 2003.

Here's the fix:

Go to your Windows Control Panel from the Start Menu and select Add or Remove Programs. Find the Outlook Junk Email Filter Update in the list of Microsoft Office updates, and remove it. Then go to Microsoft Update and it should auto-detect the latest Junk Email Filter update, which you can then reinstall. Your junk mail filtering should now work again.