How to clean a computer

1

So my friend’s computer is really infested. Eg: i google something and the results show upé when i click on any of the links in the results, i do not go to the site i want to visit but another one trying to sell something. Yesterday i wanted to go to sun’microsystem website. So i typed sun in google. When i click on sun’s hyperlink in the results, i did not went to sun site but ended on a website selling some sun protection lotion. Further i copied the link in the adress bar and the same thing happened. what can i do to fix this?