Can you jog the axes? If so then Mach3 is probably working OK and you shouldn't need to re-install.
The G-code interpreter expects to see and recognise a limited range of characters, except in any comments section of a line (a "block" in g-code speak. If you have any other character you would probably get an error like the one you see. It could even be a non-printing character. That's why I suggest you post the code so we can see if there is a problem.
If there isn't a spurious character then re-install.