I launched IE, wherein Yahoo is my home page, and despite my pop-up blocker, the first thing that happened was a pop-up, telling me that it was INSTALLING Spy Crusher. I cannot deinstall this.
Yahoo didn't install it, It most likely came from something already in your computer somewhere, from a previous web browse. Opening your browser gave it access to the internet to install itself. Or your browser has been hijacked and your homepage replaced with a page that LOOKS like yahoo, but the links go somewhere else. You need to run an antispyware utility and a virus scan.
welcome to the shady world of spyware, a for-profit business.
