Microsoft Security Essentials detects a threat but can't remediate it

When Microsoft Security Essentials detects a potential threat that's hiding inside a compressed file with a .zip file name extension or within a network share, it tries to deal with the threat by quarantining or removing the threat.

Symptom

You might receive a notice that the Security Essentials program wasn't able to apply your actions.

Cause

In most cases, this problem occurs because Security Essentials doesn't have access to the location where the infection is located.

Solution

Remove or scan the file

If the detected threat was in a .zip file, browse to the .zip file, and then either remove the file or scan it by right-clicking the file and selecting Scan with Microsoft Security Essentials. If Security Essentials detects additional threats in the file, it notifies you about these threats and enables you to choose an appropriate action.
If the detected threat was in a network share, browse to the network share and scan it by right-clicking the file and selecting Scan with Microsoft Security Essentials. If Security Essentials detects additional threats in the network share, it notifies you about these threats and enables you to choose an appropriate action.
If you're not sure of the file's origin, one of the best solutions is to run a full scan on your computer. (For more information, see Scanning for viruses, spyware, and other potentially unwanted software.) A full scan may take some time to complete, but it makes it possible for Security Essentials to look for the source of the infection and clean it.

Note:
If a full scan fails to detect or clean the infection, contact Microsoft Security Essentials immediately by submitting a case online (for more information, see How to submit a case online).