Just a query on your generalisation of Windows programs.
Did you try Flashcut or DeskCNC.
The reason I ask this is that both reply on a seperate signal generator and not windows timing.
One thing that strike me about CNC Pro from reading the user group on Yahoo is that it doesn't seem to be supported anymore.
Nothing is being added and calls for simple things like tool offsets are ignored.
I don't have an axe to grind as I don't use any of the low end programs but I find that reading the user lists of people who are actually using the programs is a far better guide.