Have you perchance installed Google Earth lately?
I just downloaded the most recent update and was a little surprised to see this at the end of the installation process (I’ve circled the interesting bit in orange):
Note how “Set Google as my default search engine in Internet Explorer” has been pre-checked for your convenience, so that if you install Google Earth you have to take affirmative action to keep them from switching your search engine too. Nice.
I wonder how long it will be before the GE installer includes options (checked by default, of course) to change your default e-mail service to Gmail and your default IM platform to Google Talk, too?