If Mach reboots, there are usually two things causing it, either its too slow a CPU (if this is less than 500Mhz, then thats why..) or you need the special driver.
If you need the special driver, then running DriverTest.exe form the mach3 folder will reboot. If running that program reboots you, runt he batch file called specialdriver.bat and try again.
If drivertest.exe doesnt reboot, then run the optimising file as Benny suggested..
Art