Download Windows binaries for GMP 4.2.1 at: http://files.sahuconsulting.com/gmp-win32-bin/.
These are Windows binaries of GMP, the GNU MP math library (gmplib.org). They were created using the "gmp4win" project/solution patches by Brian Gladman, found at: http://fp.gladman.plus.com/computing/gmp4win.htm.
The project settings were slightly modified to make it build properly on Visual C++ Express 2005, so that they operate out-of-the-box on machines with .Net-2.0 installed. No code was changed. An archive of the exact source package used to create these binares is available in the same folder.