don, it sounds like either you've been hijacked by malware or you hit a wrong button.
I literally just went to Download OpenCPN | Official OpenCPN Homepage
and clicked on the prominent Windows logo. A pop-up asked if I wanted to save or run the setup file, I saved it then ran it. opcn installed (over my older edition) and I took all the defaults. There was never a hint of Yahoo or any other browser modification. No sign of one in the installation log either.
If you didn't hit a wrong button, I'd suggest a couple of malware scans.