Microsoft is sending condolence letters to Windows 98 and Windows Millennium users saying that that it will cut off all support for the aging operating systems in July 2006. No other security updates will follow after this date.

Here's the official reason cited by Microsoft:

Microsoft is ending support for these products because they are outdated and these older operating systems can expose customers to security risks.

Microsoft is asking customers who are still running Windows 98 or Windows Me upgrade to Windows XP, as soon as possible.

Visit Windows 98 Homepage

Source: Microsoft Support | TechWeb