The User Account Control popup is informing you that the MRT.exe executable file, created by the verified publisher (using the signature of the valid certificate the file contains) Microsoft Corporation. If you wish to verify this even further, you can
click the "Show information about the publisher's certificate" and view the detailed certificate information.
The MRT.exe file is run monthly by the Windows Update process in order to confirm that no well known malware has infiltrated your computer. If it has, a separate popup would display telling you what it had detected and whether any action was necessary.
However, in most cases the program will simply run and exit quietly, with no additional display if nothing of concern has been found.
The true question here is why this normal monthly operation has changed to require your acceptance via the User Account Control notification. If it doesn't stop after accepting it once to allow it to operate, then you might check the UAC settings by selecting
"Change when these notifications appear" to see whether it is set to the highest selection of always notify, which is a bit too extreme in most cases.
It's possible that the Microsoft Malicious Software Removal Tool has actually detected something on your system, which is why it's attempting to make changes, but it's impossible to know if that's the reason for the notification unless you allow it to perform
whatever operation it's trying to complete.
Rob
