Browser Virus?

Hi there,

I believe that I have some sort of virus on my Laptop (windows 8.1).
When I go to download from a site - such as google chrome, or Libra Office, the search appears to create false links which mimic the real sites.
I have then clicked on these links thinking they were safe and my Avast flares up recognising it as a bad download, or in the case of the chrome download it sends me to unsafe ad websites to confirm installation.

Any idea how I can get rid of whatever it is that is causing this?

Thanks,

Olivia