The new Micro$oft incoming browser, IE 7, will be more secure. It requires WinXP sp2 (ie reserved to paying customers), and even the futur Microsoft windows (codename Vista) to get the full features set.
If they work as intended, those improved security features will break some websites. You can find security compativility issues in IE 7 [1] with a Microsoft compatibility tool [2].
Refs
- IE blog [3]: Security and Compatibility with IE7 (preliminary compatibility tool) [4]
- MSDN [5]: preliminary compatibility tool [6]