Xming is the leading free unlimited X Window server for Microsoft Windows (XP/2003/Vista). It is fully featured, light and fast, simple to install and because it is standalone native Microsoft Windows, easily transported [WWW]portable as a Pocket PC X server. It is totally secure when used with [WWW]SSH and optionally includes an enhanced PuTTY Link SSH client and a portable PuTTY replacement package.

http://www.straightrunning.com/XmingNotes/

http://sourceforge.net/projects/xming