I have AMD 3200 Athalon with XP and also Intel P3 1Ghz win2k and not had any issues with Mach 3 shut down - all versions. I have laptop Intel Centrino and didn't install the drivers so not sure about that.
Had lots of hang-ups before I found the Mach3 inputs config had a random reference to Port 2 that was not enabled - kept crashing when the mouse was moved! Took months to find!
When your computer is slow, use Ctl Alt Del, select task manager and processes and see if Mach3 is still running - and CPU usage? Might help see what is going on.
By the way, did you optimise running processes? XP, I did not, Win 2k I did - helped a lot with performance but take care...