Even though Windows may be the platform that everyone loves to hate, the simple truth of the matter is that it still has market share somewhere north of 90%. Additionally, when it's all said and done, Windows is actually a fantastic platform to develop software on today. Products like Visual Studio 2005 and the .NET Framework 2.0 make it super-easy to create professional-looking applications with a minimal amount of effort. Of course, I am biased on this matter, so take my opinion with a grain of salt, but still keep in mind that if you want to write rich client applications (fat apps?) you will probably end up targeting Windows.
I'll probably be saying a lot and linking quite often to information on creating great-looking Windows apps in the near future, but there is no better place to start than the Windows User Experience Guidelines. Take this document to heart, as it contains a great deal of very useful, valuable information that can help differentiate your application in the marketplace.