By ``XWin'', do you mean XWindows or XWin32?

XWindows is a network protocol and an API. Both are free of charge, although some implementations (usually non-Unix ones) do cost money. Solaris does have a X Windows server for both SPARC and Intel platforms, as do 99% of Unix-based OSes (Mac OS X is a good example of the 1%).

XWin32 is a non-free commercial XWindows server implementation, but it has a very nice demo usage that very handy for those of us that need an XWindows server on an MSWindows machine for an hour or so once in a while.

(There are also some totally free MSWin32 XWindows servers out there, I think, but I haven't really looked in a while.)