Mach3 is a 32 bit system program, unless you are using an external motion controller.
Quote from web site.
Software Requirements
Mach3 Minimum Recommended Requirements:
*Use of Parallel Port for machine control only on 32-bit versions of windows.
32-bit Desktop – Parallel Port Requirements
32-bit version of Windows 2000, Windows XP, Windows Vista, or Windows 7
1Ghz CPU
512MB RAM
Non-integrated Video Card with 32MB RAM(Large G-code files, especially 3D files will require a video card with 512MB RAM or higher)
Laptop, 32 or 64-bit Desktop – External Motion Controller Only
An appropriate external motion controller (options can be found on the Plugins page)
Windows 2000, Windows XP, Windows Vista, Windows 7, or Windows 8
1Ghz CPU
512MB RAM
Video Card with 32MB RAM(Large G-code files, especially 3D files will require a video card with 512MB RAM or highe