I'm late to this, but if I understand your problem correctly, you could tweak your firewall to auto-reject outbound packets being sent to Yahoo or Yahoo subdomains. You could also do this with a proxy intercepting all the requests your browser makes. Look at the headers and bodies of the packets and see if you can get a sense of what's going on in them to find out origin or referral codes for a possible abuser.

The next step is to remove the malware from your system. I'm assuming that you're using Windows (I don't think that's a dangerous assumption, as I've only ever seen toolbar malware and related on Windows). You'll want to google-fu the details you found in the packet headers. If that turns up nothing, dig around in your registry.