With Internet Explorer 7 Beta 1 set to debut next month, Microsoft has confirmed Windows 2000 will not be adopting the new Web browser. IE7 will require Windows XP Service Pack 2 due to internal security changes that rely on Microsoft’s latest operating system release.

Quoted:
It should be no surprise that we do not plan on releasing IE7 for Windows 2000. One reason is where we are in the Windows 2000 lifecycle. Another is that some of the security work in IE7 relies on operating system functionality in XPSP2 that is non-trivial to port back to Windows 2000.

Besides, support for both IE 5.01 SP3 and IE 6 SP1 on Windows 2000 SP3 will expire. Users running IE 5.01 or IE 6 SP1 on Windows 2000 should upgrade to Windows 2000 SP4 in order to continue to receive security updates.

[Source - IEBlog]