There is a serious vunerability in JPEG file parsing in Windows operating systems, which would allow attacher to execute arbitary code. Patch is already available. However, these opensource patches are properly preferred. It is indeed serious - imaging evil websites with JPEG images with exploits? Delivered to you in Outlook which automatically previews? Via MSN Messenger? Yuck.
Update: Originially I was only going to put it into the "Quickies" category - until I actually tried to do a Windows upgrade on the Win2k notebook that I am using at work. Not just I have to updated my Windows, I have to go to another website to upgrade the Office 2000 installed as well. After many clicks, Office upgrade failed for no apparent reason. Am I secured or not? What a mess.
Update 2: Ironically, less than a day later serious vulnerability has been discovered and fixed in Mozilla's bitmap parsing code as well. Download the latest version and you will be secured. I think I'll be sticking to browse Internet using w3m under a text terminal for a while.