Trying to figure out what is and is not supported, and to what extent, is certainly difficult if you go to the appropriate MS sites, so I may be wrong about my earlier assertion.
From an article on CNet by Paul Festa:
Microsoft this week reiterated that it would keep the new version of Microsoft's IE Web browser available only as part of the recently released Windows XP operating system, Service Pack 2. The upgrade to XP from any previous Windows versions is $99 when ordered from Microsoft. Starting from scratch, the operating system costs $199.
It's a problem that people should have to pay for a whole OS upgrade to get a safe browser," said Michael Cherry, analyst with Directions on Microsoft in Redmond, Wash. "It does look like a certain amount of this is to encourage upgrade to XP."
Microsoft affirmed that its recent security improvements to IE would be made available only to XP users.
"We do not have plans to deliver Windows XP SP2 enhancements for Windows 2000 or other older versions of Windows," the company said in a statement.http://news.com.com/Microsoft+to+secure+IE..._3-5378366.html
If this is so, I bet the first critical security patch not available for the 50 percent of MS users will bring the wrath of heaven down on the "trustworthy computing" folks at MS.