[Stop It]

Ah, good old search hijacks.

if you're sure you're clean of any nasties, make sure the shortcut used to launch Chrome is clean.

Right click the short cut and click Properties, make sure the Target field doesn't have anything after the Chrome.exe bit. It's a common tactic of search hijackers to insert the URL of the nasty there, although Yahoo isn't generally used in that way of course.

No idea why it would show under each new tab however, that's just weird. Make sure the default New Tab isn't changed in Chrome settings if you haven't already.