Support for Windows Vista without any service packs ended 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 via Windows Update.
More info here:
http://windows.microsoft.com/en-us/windows/help/what-does-end-of-support-mean