Windows API Functions by Release
A really handy reference for those developing Win32 apps that target different operating systems is the Functions by Release topic in the MSDN Library. If you want to know what's new in a recent OS, or what APIs you can use with older OSes, this is a great place to find out.
The Functions by Release topics give you a list of Windows API additions grouped by operating system release. You can also link to a list of functions in alphabetical order, and by category. I can't say if the list is 100% complete, so check the API listing by category to doublecheck for a specific API.
The Functions by Release topic covers these Microsoft operating systems: