Something is making ~15GB of .TMP files every few hours

[avery]

Try a couple of things:

1) Do a full scan for malware with malwarebytes antimalware. Clean whatever it wants to clean.

2) Go to MSCONFIG and disable all startup items except your antivirus. Reboot, then observe if the problem persists. If it doesnt, you'll need to re-enable the startups one by one to isolate the culprit.

I believe that the culprit is Microsoft Security Essentials.

Have you disabled that service to see if it's the cause?

Yes - it is not the cause.