We're at the end of "Mainstream" support for SQL Server 2000, which is April 8th, 2008. If you're still using this version in your enterprise, you need to plan to move off of that version platform starting now. Many folks are planning to "jump" from SQL Server 2000 to SQL Server 2008, but if you need to go sooner than the release date, the migration from 2000 to 2005 works well (if you plan properly) and 2000 to 2008 is a similar planning effort.
Just because "Mainstream" support will end, you can still get certain kinds of support for the product. More here: http://support.microsoft.com/lifecycle/?p1=2852