It might be wise to steer your employees towards Yahoo! as the search engine of choice and give the company network an extra layer of security.
While you're at it, consider turning off the "cookies" feature on as many work stations as possible. Cookies are the little piece of code web sites attach to the computers of site visitors. It cuts out a lot of risks, but it also cuts out a lot of surfing features most folks can't live without - like logging into a frequently visited site, accessing content at a trusted news site or shopping online at a favorite store.