Suspecting to have a virus

Hello,

I have been using small office suite since 2 years and i was quiet happy with it as i never had any issue of viruses or spyware.. until recently, i found that my has started acting up and it is opening some websites automatically.. i had scanned my system with kaspersky, norton, Bitdefender for viruses and superantispyware, malwarebytes, spybot search & destroy for spywares. none of these had found anything in my computer but still random websites opens in the browser without any reason. i have tried malwarebytes anti-rootkit too but it didn't helped me. i made some security scans of this computer if any of my ports are opened as i suspected to have a hacking] attempt too. but even the port scan and firewall stealth scan came up negative. i know that my computer is still infected as the internet speed has gone down, programs stops working automatically. i have checked my hardware but nothing wrong in that too. it is making me confused. i am using outlook for my business and i have set kaspersky as a scanning agent for incoming and outgoing mails. i don't suspect any mail worm though. any suggestions, please?