So if you are running either Windows Vista with no service packs or Windows XP with SP2 then read on.
Support for Windows Vista without any service packs will end on April 13, 2010.
Support for Windows XP with Service Pack 2 (SP2) will end on July 13, 2010.
If you're running one of these versions after support ends, you won't get security updates for Windows.
“An unsupported version of Windows will no longer receive software updates from Windows Update. These include security updates that can help protect your PC from harmful viruses, spyware, and other malicious software, which can steal your personal information. Windows Update also installs the latest software updates to improve the reliability of Windows—new drivers for your hardware and more” read more
For more info see End of support for Windows XP with Service Pack (SP2) and Windows Vista without service packs which has information about how you can help you find out what you are running
I highly recommend that you should always install the latest service packs from Microsoft for your version of Windows. You can download and install these through Windows Update.
For information about using Windows Update in Windows XP, see Stay up to date automatically.
For information about using Windows Update in Windows Vista, see Install Windows updates.
For information about using Windows Update in Windows 7, see Install Windows updates.