See this recent discussion on slashdot:

The latest from the oh-so-brilliant MS "Chief Software Architect" is that all of the security flaws in their Internet Explorer browser are due to installing third-party apps. Really? Is that so? Then why is it that when I (or anyone else) switches to an alternate browser, 99.9% of their spyware/adware/malware problems vanish into thin air? Coincidence? I think not.