Prior to 2004 there was no Windows Firewall product available for then current Windows 2000, Windows XP, or Windows 2003 Operating systems. Instead, Systems Administrators needed to use something that Microsoft called an IPSec filter to obtain firewall functionality in these operating systems.
In 2004 with the release of Windows XP Service Pack 2 and Windows 2003 Service Pack 1, the Windows Firewall debuted for each operating system, and has since been bundled into Windows Vista, Windows 2008 and Windows 7. The IPSec filter remains the only way to achieve firewall functionality in Windows 2000, which is no longer supported by Microsoft.
Microsoft maintains an knowledge base article on how to configure IPSec filtering which may be useful for administrators of Windows 2000 systems that need this protection.