This is a program that monitors memory marking some memory locations as non-executable, therefore if a program tries to run code from this memory, the DEP will close the program and display the DEP message and naming the program causing it.
There are many reasons why this message was displayed, and one could be that you have malicious code that was trying to run (this is the purpose of this program) or it could just be an error cause by an old driver you have of your PC, DEP was only introduced from XP service pack 2 onwards so if you've upgraded from an earlier version, this could be the reason, it could also be that your using an older application on your PC.
If you are confident that you haven't anything harmful on your PC and it continues to happen you can change the settings of the DEP, by default it's set to only allow essential windows programs and services to use these memory locations, but you can switch it to allow all programs except these you select. To do this:
1. right click on My Computer and select Properties
2. select the Advance Tab and in the performance section press the setting button
3. a new popup will appear, on this select the Data Execution Prevention tab and you will see the 2 options described above, select the one you require (may be worth checking to see if you have the second option already selected as this could actually be causing the problem as you mentions it happened to Explorer)
Note if you are not sure your PC's clean or have any concerns in general please PC check.
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