I have tried to download and run Mach 3. Every time I try to RUN, it gets halfway thru and everything goes black and the computer locks up. I'm using a DELL Optiplex GX620 desktop with Windows XP 32 bit. I have tried this on my laptop and everything downloads and runs as it should-except for running my CNC machine smoothly that's why I'm trying to put it on my desktop computer. What's going on?

You may need to do a clean install of your computer as likely there are loads of programmes on it , one of which may not be  happy about Mach.
Hood

Also the onboard graphics can be a problem unless you have a seperate PCI graphics card fitted and selected in BIOS.

Tweakie.