How many successful reasonably known startups run on windows?

Well-Stack Overflow, Serverfault and all the other things Jeff Atwood is involved with runs Windows.

Myspace does too: http://highscalability.com/myspace-architecture

I've read that eBay's Java front end runs on Windows (and still runs on it due to general cluelessness; their capacity management problems have to do with the latter).

Does Microsoft count?

Writely used Windows and C#. (Google bought it in 2006, now it is Google Docs.)