If you look in the Mach3 folder you will see a file called Mach3Mill.xml, that is assuming you dont have a custom profile set up, if you do the XML will have the name of that profile. Anyway all you do is right click on the xml and select copy, browse to some other location on your hard drives such as My Documents, Desktop or whatever and again right click in a blank space and this time select paste, that will be your xml copied
If you have any custom screens or use keygrabber be sure to back them up in the same manner as you have just done with the xml.
Hood
EDIT
forgot to say when you have reinstalled Mach just reverse the procedure to put your xml back into the Mach3 folder, you will be asked if you want to overwrite the default one, choose yes.