Here are a list of open source X Window System for Windows
Xming is the leading free unlimited X Window server for Microsoft Windows (XP/2003/Vista). It is fully featured, small and fast, simple to install and being standalone native Microsoft Windows, easily transported portable as a Pocket PC X server.
Cygwin/X is a port of the X Window System to the Microsoft Windows family of operating systems. Cygwin/X runs on all recent consumer and business versions of Windows; as of 2003-12-27 those versions are specifically Windows 95, Windows 98, Windows Me, Windows NT 4.0, Windows 2000, Windows XP, and Windows Server 2003.
Cygwin/X consists of an X Server, X libraries, and nearly all of the standard X clients, such as xterm, xhost, xdpyinfo, xclock, and xeyes. Cygwin/X, as the name implies, uses the Cygwin project which provides a UNIX-like API to Xlib and X clients, thereby minimizing the amount of porting required.
Cygwin/X is licensed under an X style license; Cygwin is licensed under a modified GNU General Public License that specifically allows libcygwin1.a to be linked to programs that are licensed under an Open Source compliant license without such linking requiring that those Open Source programs be licensed under the GNU General Public License (see the Cygwin licensing page for more information). Source code and binaries for both projects are freely available.
Other interesting things you may want to know
WeirdX is an X Window System server in pure Java under GNU GPL.
It is derived from WiredX-Lite and will run on any platforms which support Java 1.1. This X server will not be suitable to exec heavy x clients, but you may find that twm, xterm, etc. will run fine on it.
WiredX.net enables access to unix applications from Windows machines via web browsers.