Basically, as planned, Microsoft ended Mainstream Support for Windows XP today. As far as I am aware, this means that Microsoft will no longer provide non-security updates to private users. As Extended Support is still available (and will be for years to come), Microsoft is still going to provide security updates for free as and when they are required. Microsoft is not planning to cut security updates for XP until April 8, 2014.
The company is also going to continue providing support both over the phone and on the web.
Basically, there’s not much to worry about. However, when Windows 7 is released, I would suggest that people start to think about moving to that rather than hanging on an earlier version of Windows that will only provide security fixes for a limited period.
The same Mainstream Support has ended for Office 2003, so users may also consider upgrading to Office 2007 or waiting until the next version of Office is released with Windows 7.
Update: It also seems to have become apparent that you need to be on Windows XP SP3/Office 2003 SP3 to continue to receive important security updates.