Why my computer is constantly telling me that I have a trojan, although I scanned?

There are windows system alerts and keep emerging tell me I have spyware, I need to purchase the security and my anti-spyware and antivirus Web sites. My sister accidentally yesterday a program called Virus Heat and other I already installed and removed. I scanned my computer with Panda Anti Virus and Anti Spyware I downloaded from microsoft.com, but nothing happened. Its really an obstacle, because I'm writing a report and I have every time something seems to click Cancel. I do not think that pop-ups, my computer, because they tell me to places where they I want to sell the software. What can I do?

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Hardware Sensors Monitor – Ubuntu 9.10