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The History tab is where you can study what actions Microsoft Security Essentials has taken as a result of software running on the computer.
You can get an overview in the following categories :

• All detected items - View of all the items that were detected on your computer
• Quarantined items - Items that were detected and prevented from running but not removed
• Allowed items - Items that you've allowed to run on your computer

Microsoft Security Essentials - History

Which items are in which category will depend upon any configuration changes you've made with respect to the actions that Microsoft Security Essentials takes upon detection of threatening items. (For example, it may be that you've decided to prevent access to items rather than delete them if you find yourself in receipt of infected documents that you can otherwise retrieve the text from - in which case you may have more quarantined items rather than deleted items).