It looks like Google's starting to drift away from its famous 'Do No Evil' motto. In it's latest version of the Google IE toolbar, Google has added a new 'feature' that ensures that Google remains the default search engine in IE's search bar. And this feature is enabled by default.
An exe called GoogleToolbarNotifier.exe is made to run at startup and it monitors any changes to the default search settings in IE. Every time an attempt is made to change the search engine (from Google), the attempt is blocked and the following message is displayed in the system tray.
According to this post in the Google Operating System blog, Google says this is a bug and that they are working on removing it.
A bug that prevents you from changing your search engine to anything but Google?! Haha!