Windows Defender Disabled by Microsoft Security Essentials

Recently, I noticed that Windows Defender was not running automatically as it had been until a week or so ago. After doing a little digging, I learned that other security programs tend to turn Windows Defender off on their installation. Since I recently installed Microsoft Security Essentials on my Vista PC, I'm concluding that one Microsoft product turned off another. A little warning would have been nice, but then there wouldn't have been a problem to solve.