Are you using a windows 3.1 disc when XP is installed?
You can't install windows 3.1 'into XP.' They are separate OS's.
Answer, Well, two separate computer professionals assured me the Windows 3.1 CD 'ought to' play on XP, and one of them spent some time checking (without success) for problems with my computer. However, I have now discovered and obtained a 'patch' (installation disc) from the original vendor, which thankfully works. So the difference in operating systems is evidently not without remedy. All the same, from blogging elsewhere on the internet, it seems the w16 subsystem (whatever that is) may be a wider problem.
Regards to all