Home page overtaken by anknow website

Hi,
Everytime I connect to the internet my home page has been over taked by this address http://81.211.105.68/index.php?v=6&aff=2623673. I do change my home page with a different on like google by going to the tools then internet option, but everytime I disconnect and connect back to the internet this webpage comes back again. I have already scanned my all computer for virus but nothing was detected.
Can someone tell me what is causing this? I will appriciate.

Thanks