I haven't had this specific problem, but I did have a program do something similar to me. I believe it was part of Comet Cursor, but it could have been another program. I would look at your system processes again and do a search on yahoo.com for any files that look suspicious. If you find one that has a record you should see results pretty quickly. Once you learn what program it is, uninstall it. To help clarify how the problem took place, changes like that occur through program installation or viruses...(both being exe type files). So you will probably have to uninstall what did it. Keep searching....
My problem was that I'd close IE and walk away. I'd walk back up to my computer and it was on web page.... I'd close the window and at random increments of time, it would spawn another page and so on. This is because someone is trying to rack up $$$ on advertising redirects...