What should I do if Microsoft Security Essentials detects malicious

If Microsoft Security Essentials detects malicious software or potentially unwanted software
on your PC (either when monitoring your PC using real-time protection or after running a scan),
it notifies you about the detected item by displaying a message in the notification area to the right
 of the taskbar.

In some cases, Microsoft Security Essentials takes automatic action to remove malicious software from
your PC, and will notify you that it is doing so. In other cases, Microsoft Security Essentials will show
you a notification that malicious or potentially unwanted software has been detected.
Click Clean computer to remove the software, or click Show details to open
the Potential threat details window and get additional information about the detected item.
If you need help determining which action to apply to the detected item, use the alert level
that Microsoft Security Essentials assigned to the item as your guide (for more information,
see Alert levels in Microsoft Security Essentials).

Alert levels help you choose how to respond to viruses, spyware, and other potentially unwanted
software. While Microsoft Security Essentials will recommend that you remove all viruses and spyware,
not all software that is flagged is malicious or unwanted. The following information can help you
decide what to do if Microsoft Security Essentials detects potentially unwanted software on your PC.

Depending on the alert level, you can choose one of the following actions to apply to the detected item:

• Remove—This action permanently deletes the software from your PC.
• Quarantine—This action quarantines the software so that it can't run.
  When Microsoft Security Essentials quarantines software, it moves
  it to another location on your PC, and then prevents the software from
  running until you choose to restore it or remove it.
• Allow—This action adds the software to the Microsoft Security Essentials
  allowed list and allows it to run on your PC. Microsoft Security Essentials
  will stop alerting you to risks that the software might pose to your privacy or to your PC.

Caution:

If you choose Allow for an item, such as software, Microsoft Security Essentials will stop
alerting you to risks that the software might pose to your privacy or to your PC.
Therefore, add software to the allowed list only if you trust the software and the software publisher.

http://windows.microsoft.com/