Security scan said severe threat/malware could not be located

I was online and an alert came up from Microsoft Security Essentials that a severe threat had been detected. I clicked on my security icon and it was spinning/searching. When it stopped, I clicked on the scan history and it said that the malware could

not be located but the result said that the threat had been removed. Do I need to worry that something has infected my computer and is floating around in there somewhere?

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