You need to get a copy of your Mach 3 folder onto a USB stick etc first.
Then set the new machine up, install Mach3, do the port test if using PP, install any custom drivers for controllers, joysticks and so on.
Now i just copy the Mach3 folder form USB and place it over the top of the new install, this will copy your settings, macros etc.
Create a new desktop shortcut if needed and should be good to go.
If using Ethernet smoothstepper controller or UC100/300 etc then these will need installing before copying over your old data.
This is on XP systems, i'm not sure if the newer Win7 setups are so easy.