Going into browsing with Yahoo instead of having chosen Google

Good sunday to others members of the community. I have a little problem: I am seeing that when placed on Google Chrome, and having specified “Google” as browser, having done even the reset recommended on google chrome´s own toolbar. So, possibly have uploaded some type of malware. Problem is, if situation due to malware, up to which extent this is related? noted this has started occuring after I downloaded an app requested for participating on a webbinar offered by a serious and reliable professional association. Any one having a similar problem? counsels? I have Norton 360 Premium edition, duly updated, Scan says all is ok. What can be the problem?