For some reason I was browsing through old emails and found some old development screenshots of Unity 2.5.
Unity 2.5 was the first version of the editor running on Windows and eventually was released in March 2009. Some development of it started at end of 2007, just after Unity 2.0 was released. First there was some serious code shuffling, converting from Objective C into C++, refactoring into platform independent interfaces and similar highly invisible stuff.
Here's the first Unity running on Windows screenshot I could find (this is February 2008):