Sorry for the delay in replying to everyone,
I ran the calibration cycle again repeatedly last night and found out where the fault was... ME!!!
I was making a mistake between MM and CM, DOH!!!!
Once I got the Z working perfectly going up and down 10mm, (the safe height), whenever I ran a file to cut a 10mm square I realised that if one of three axis worked correctly I must have been doing something wrong. So I started over again on the X and Y axis and got it right. By the time I finished it was travelling 2000mm on the X axis repeatedly and 1000mm on the Y axis repeatedly.
Before I had to replace the PC the X and Y 'steps per unit' was set to 710 now it is around 29. I wanted to be accurate when I posted the new value so I went back to the machine to check and now every time I start MACH3 the machine crashes!!!! Not something one wants just before bed!!!
I am running a 64Bit PC with 8GB RAM with Windows 7 (32bit) installed and as I said it was running fine for a few days...
The PC is not new but it was only used for about 3 months before I put it on my CNC machine last week...
There is always another problem isn't there? Any ideas please?