Depends on how you look at it. Windows Server 2003 is not a bad server OS and properly written and deployed ASP.NET application will be able to handle TONS of requests. Two reasons: native code execution (.NET caches compiled code in memory) and very efficient threading. This thought may be unpopular but it does not make it less true.

However, as a struggling one-two person startup, you probably will not want to pay crazy prices for dedicated windows hosting (Win2003 server+MSSQL). And shared Windows hosting is always crap: overloaded servers, terrible performance, etc.

Basically, Windows platform is not very startup-friendly: you develop slower (thus will take you longer, i.e. you'll bleed more money) and later it will cost you more to host your app. Besides, a proper set of Visual Studio + MSDN subscription is not exactly cheap.

For well-funded startups .NET makes more economic sense: easier to hire for and performs/scales well (myspace).