Hi guys, please I need your help on how to restore my HP Pavilion back to its default OS

It's a UK used HP pavilion which I have been using for months now, it works fine and is on windows 8 until some days ago when I notice my google search keeps giving me results from Uk, even wen I access yahoo, it takes me to Yahoo UK, my Facebook also was not login anymore as it tells me am trying to access it from a different location, my emails too, the same thing.

Initially, I tut it was a problem with my cookies, I den cleared all cookies and browser history, yet it's still remain same, I den concluded to get a new OS to install, den I got windows8 professional hoping after my clean install, d problem will b solved.
The problem was still the same, infact now my system slows nd hang somtimes.

Please I want to know if there is a way I can restore my default windows8 dat came with d system?. Also if there is a solution for the location issue.

Any suggestions will be appreciated