Jim, looking up "The Win 16 subsystem has insufficient resources to continue running" in Google, it seems to be a design feature of Windows XP. For instance: http://www.vttoth.com/wow32.htm (the description is good but don't attempt the "solution" that that web page proposes: it sounds dangerous).
As far as I can work out from the description, the problem depends on what other programs you have been using on Windows XP before you started the 16-bit program, and how you were using them.
The only thing I can suggest is that you upgrade to Cardbox 3.0, which is a 32-bit program and is therefore immune to Microsoft's 16-bit bugs.
You are quite right - on both occasions it happened, I had a number of programs, browsers etc open on XP - and attempted to open Cardbox simultaneously.
I will indeed uograde - in all honesty, I was not aware there was a Cardbox 3.0 until I posted the above query and staretd reading the rest of the site. I thought the version I had was pretty comprehensive, but Cardbox 3 sounds even better, and of course, as you say, being 32-bit, this should solve the 16-bit problem.
Incidentally, I too noted the suggested solution on http://www.vttoth.com/wow32.htm but thought it did not have air of confidence about it!
Thanks again - what a wonderful program Cardbox is!