1) Try disabling everything in the bios your not using, like sound, floppy drives, network. Everything but video. In windows you shouldn't install any drivers for anything but video, or hardware that you need.
2) Read the portion of the manual about installation, and try the special driver.
3) Try a separate video card instead of onboard video.
Note that a very small percentage of PC's just can not run Mach3. It's possible that you may have one of them.