OK, it gets even more strange....

I used a second laptop, installed the software, copied the program, and it works just fine. Now, the original laptop would upload and download to the old MX-850 only. Laptop#2 will work with both units. Software versions are the same. Was there some sort of update to the Mx-850 that would cause a laptop with Windows 2000 not to work any more? Both units are using standard serial cable, no adapters.