Something is causing issues, and it's not XP.
I run Mach3 on a 1Ghz PIII with 256Mb of RAM.
With just Mach3 running, the PC only uses about 60Mb of RAM.
If I run driver test I get an error report 'Cannot find vectors' 'Pulsing too fast'.
You need to resolve this issue.
First thing is find the Mach3 optimization guide, and go through the steps one by one.
Actually, the first thing I would do is change the PC mode from ACPI to Standard. This may require re-installing Windows. I have a PC that would not run Mach3 in ACPI mode, but works fine in Standard mode.
Personally, I would consider 1Ghz to be the absolute minimum, based on my experience. While it works fine, there's very little margin for error.