It depends on who your market is. I can tell you one thing, your market will never consist of the whole of mankind. So that fact that maybe 25% of the people on the planet don't use Windows shouldn't really matter to you.
What matters to you is how many people in your market use whatever OS? If you're writing a business/financial application and you only develop for Windows, then you're probably only leaving out about .05% of your market (because when is the last time you heard of an Accountant that uses Macs or Linux?).
However, if you're writing a program for producing music (like FruityLoops) and you write Windows only then you're probably leaving out more like 50% of your market.
