Dateline July 2006: Microsoft discontinued update support for Windows 98.
Dateline October 2006: Microsoft discontinued updates for Windows XP Service Pack 1. October 10th's SP1 updates were the last of all public assisted support.
Service Pack 2 was released in September 2004. So, if you're still running with SP1 - it's really the time to update.
And another thing. We mentioned this last month, but it bears reminding that Microsoft will be pushing Internet Explorer 7 as an automatic update rather soon. Perhaps during October. According to the details that we've read, the update will prompt for confirmation before installing. If you have Automatic Updates enabled, be ready for the prompt; backup your settings and favorites. And maybe install a second browser if you don't have one already.
A few weeks ago, we tested the install of IE7 RC1 on an adware toolbar loaded IE6. The toolbars caused some buggy behavior, but the install completed itself with no trouble. Still, that's a lot of browser installs that will take place when the update is released. Hopefully it will go smoothly.
If you're an admin and your domain isn't ready for IE7 - there's a toolkit available to disable delivery.