In a few months to come, Microsoft is going to end its support for some products and services, therefore it is recommended that people running these software need to update them before the set date for closure.
On April 13, 2010 support will end for Windows Vista with no service packs (Vista RTM and Vista SP0), and Windows XP SP2. If you are running these versions after that April 13 you will no longer receive updates or support.
What do do? Apply the latest service pack (SP2 for Vista, SP3 for XP) or buy a new PC running Windows 7.
On July 13, 2010, Windows 2000 in all service packs will reach the end of it's "Extended Support Phase" meaning no updates anymore. (Probably also the end of all support for Internet Explorer 5, currently supported on Windows 2000 only).
On July 13, 2010, all editions of Windows Server 2003 will be moving from the Mainstream Support phase to the Extended Support phase. This means the end of "no charge" support and Microsoft will no longer be providing new non-security hotfixes. You will need to open a paid support case in order to get support from Microsoft.