I have windows 7 on virtualbox, because our software depends on a vendor who only provides a COM interface to their API. Thus we need some windows glue.

Also, there is some really neat software built for windows and it comes pre-installed on virtually any computer you intend to buy for an office that people keep it as their main OS. So if you intend to write a service that runs on a client's computer, it almost certainly has to be usable on windows.