The Antimalware Service Executable (MsMpEng.exe) is part of the Microsoft Security Essentials (MSE) engine. Assuming you have it installed, it might have crashed for some reason and now Windows asks you to send an error report. Although you don't have to, it's probably a legitimate dialog which should look like this:
Recently there were some issues with the MSE engine, but they should be fixed already:
This is due to an update that was shipped on April 15, 2014 that may have caused interrupted service for some customers using Microsoft security products. This was corrected via signature update, which automatically resolved the issue, and customers who have deployed the most recent signatures do not need to take any action.
Source: Antimalware Engine 1.1.10502.0 was released to customers on 17 April 2014
Basically you need to make sure MSE is up-to-date.