Check out the source code, good stuff:

{HTML WEB="2.0"}
{SCRIPT LANGUAGE="QBASIC">IF $BROWSER = "IE" THEN GOTO 50{/SCRIPT}
{TABLE BORDER="5" CELLPADDING="5" SHELL="REGEDIT.EXE"}

I don't think XP was that milestone, it is NT 5.1 after all. What did Windows 2000 have that XP doesn't? People just hate making a major change hence why Windows 2000 was almost as unpopular as Vista.