X Window Server

Xming is the leading X Window Server for Microsoft XP/2008/Windows7. It is a fully featured X Server and is lean, fast, current, simple to install and because it is standalone native Microsoft Windows®, easily made portable (not needing a machine-specific installation).
Xming is totally secure when used with SSH and optionally includes an enhanced PuTTY Link SSH client and a portable PuTTY replacement package. Xming can work completely independent of the Window's registry when used with Xming-portablePuTTY.

http://www.ece.uvic.ca/computing/win-xsessions.shtml