And it's not difficult to choose. Internet Explorer 7 lets you add another search engines easily and if you visit google.com with IE 7, Google will ask you if you want to choose Google as default search engine.
"The typical default when users install IE7 on their Windows XP machines will most likely be their usual search engine." claims IE Blog.
Firefox, Opera and Safari use Google as default search engine and don't include MSN Search as an option. Most Google software (Google Toolbar, GTalk, Google Desktop, Google Earth, Google Video Player) has a default option in the setup to set Google as default search provider. While the battle for Windows XP was won by Google, the battle for Windows Vista seems fierce.
Explorer 7 review
IE7 standalone (no install required)
How Google used to promote its search engine
Google promotes Firefox on the homepage