Do I have a virus and what can I do?

My computer gets many pop-ups and Spyware Guard keeps showing up. I've removed all the files of Spyware Guard at the Control Panel but it still shows up. I have McAfee. I know it's not good, but it's protected my computer for a long time, now. I've download Spyware terminator and Smart Pop-up Blocker, but I still get pop-ups and Spyware Guard showing up. I have to restart my computer 100 times after turning it on for it to start working. Is there anything I can do that is free and effective?