Microsoft have posted on their blog some important dates for Windows users. If you are using Windows XP Serivce Pack 2, Windows 2000 (all versions) or Windows Vista RTM (ie with no service packs) – this affects you.

• On April 13th 2010, Windows Vista RTM will loose support. To continue support, you’ll need to (at the very least) download Service Pack 1. You’ll be better off if you download Service Pack 2 as well.
• On July 13th 2010, all versions of Windows 2000 will loose support. You must upgrade to a newer version of Windows (ie Windows 7) to regain support. Windows XP Service Pack 2 will also loose support on this date. You must upgrade to Service Pack 3 to regain support, but bear in mind that support for SP3 will end soon.]

That should tell you all that you need to know. If you need to get your hands on Service Packs, check Windows Update.