If you know how to change registry,you probably can access and change the default search engine through "regedit".
I have found this page:
http://web.wenxuecity.com/BBSView.php?SubID=computer&MsgID=163853
may give you some hint.
It appears that the default search engine registry is located in
[HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\SearchScopes]
you can delete and change default search engine from there.
IF YOU DO NOT KNOW HOW TO CHANGE REGISTRY,DO NOT TOUCH IT. OR ELSE YOU WILL MESS UP YOUR WINDOWS.