In a "closed source" system like Windows XP, it is not possible for us to know for a certainty by examination of the code, but your mysterious solution to your problem of running old FOX on XP parallels my personal experience.

What happens inside the XP caching seems to dramatically affect the operation of older programs -- some of which were written for dedicated DOS machines -- where the program thought it owned the CPU, most of RAM and certainly 100% control of the files which it has open.

I personally believe that it is this 100% control mind set on the part of the older software and the real world of XP's real control of all the I/O that is the basis for the problem. Again, that's my belief on the topic, not something I can point to in the code.