Conditional Compilation for Specific Windows

[Jonathan Marler via Digitalmars-d-learn]

I'm looking at the Windows multicast API.  It has different 
socket options depending on if you are on Windows XP or Windows 
Vista (and later).  Is there a way to tell at runtime which 
version of windows you are on? Note: I'm specifically talking 
about runtime because I want the same binary to run on all 
windows versions so I have to support both and determine which 
one I am running on at runtime.