Just come across this site:
It's saying that Windows Security Essentials should only be used as a secondary anti virus program. Is this true? It sounds like a legit URL and I'm not really too sure what they're trying to sell otherwise. It's also the first result returned by google when I search for "Windows Security Essentials"
I always thought having two anti virus programs running at the same time on your computer would be a bad idea as they would conflict with each other and also be bad for performance.