What do I do with this virus?

[terenkleon]

I think my laptop has a virus. It is a Dell xps with windows 7. This windows security window popped up and said that my computer was infected but it required me to buy before it would do anything. My mcafee said it found and deleted a Trojan. The windows security box kept popping up and wouldn't allow me to do anything so I shut my computer down. Now it will go on but I cant get to the log in screen. It just turns on and the screen turns blue and says something is installed right. I have already tried scanning it and setting it back to the settings from another date but nothing is working. What should i do?

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[Guest swizzer2012]

Maybe should better reformat your laptop, it could help. Just back-up your files.