What operating system do you have on your computer. I have xp and installed mach 3 on all of mine and never once had any problems.

per.

I have a lathe that I use Mach3 on, I love it but there is some kind of quirk where sometimes it will just freeze the computer.

What does the Time in Int and the Frequency say on the diagnostics page? You will likely have to load a mill screenset to see these as I dont think they are part of the standard Turn screenset.
Hood

I don't remember it crashing during the driver test.

I have even tried un-installing, re-installing. Also have run the driver test more than once.  The driver test looks O.K. not perfectly flat but pretty good.

The strange thing is, This PC worked flawlessly until the harddrive failed. i installed a new HD, Loaded win XP. Optimised for Mach.