Microsoft make money... the security of Windows is paramount to Microsoft making money. Windows makes Billions of Dollars a Quarter... if they want to keep it that way they will make it secure.
The problem is... over XP's lifetime which will be too long (no Linux OS from 2001 is used anymore because it would be less secure than Windows) the Internet has changed vastly. Spyware, Adware, Trojans and Viruses on such a scale could not be presumed or dreamed of when Windows XP was put into Beta, not even when it was released to a certain extent. Longhorn is made on that basis or dreadful Internet problems. It will be secure or nobody will buy it and Microsoft knows this with the Linux threat looming ever closer. (Hence my rousing Troop Prep-Talk in my Signiture.)
A Knight is sworn to Honour. His heart knows only Virtue. His blade defends the helpless. His might upholds the Weak. His word speaks only truth. His wrath undoes the Wicked.