Sounds like your computer has a malware called a browser hijacker. It redirects you to advertisements or sites which may covertly try to send more malware.
You need to track down this problem right away because it could grow. Run a virus checker, preferably more than one.
Examine all processes running in your system, by opening Task Manager. (Consider getting a free utility called SysInternals Process Explorer.)
Look for any identifying behavior in the browser hijacker. When you find out its name then you can do an internet search in hopes there's an easy way to eliminate it. It can ruin your system to a degree you'll need to reformat your hard disk and reinstall everything.