Whenever I use a search engine and click on one of the result sites it takes me to basically a pop-up ad?

Whenever I use google or yahoo and try to click on one of the sites shown up it redirects me to some other advertisement site. Also, whenever I try to open up My Computer or the control panel it says that my windows explorer has encountered an error and has to stop running. This problem just started today and I have no idea how it happened. I already have antivirus software on it, which I just ran and came up with nothing. Got any suggestions for a computer illiterate person?