This is much ado about nothing. Here is a quote from the referenced TechCrunch article:

"The results also say that web server is Microsoft-IIS/7.0. That doesn’t sound right, so what gives? Well, it turns out that because Gates is using Akamai to mirror his sites’ content in the event of massive traffic (or more specifically, something like a DDoS attack), this data is being filtered through there. Akamai uses Linux for its servers, so that’s what OS is being passed back to Netcraft."