Occasionally (maybe once a day), when I download any file with my browser and first try to open the download, my Windows 8 box CPU spikes to 100% for a minute or more. The file doesn't open until this spike subsides. (Not to mention that I can't do anything else which accesses the disk until it finishes.)
In looking at the Performance Monitor, I see that the process consuming disk is MsMpEng.exe, which as I understand it is Windows Defender.
I see that it is accessing a whole bunch of "Package" files in a Windows\System32\catroot directory.
Does anyone know what it's doing here, and whether I can prevent and/or optimize it so I don't get these lengthy disk-blocking operations? My naive guess is that it's loading its dictionary of malware detection/correction definitions from disk into memory but I'm not sure.
Windows Defender, Windows' built-in antivirus. You can disable it from its settings.