Final check in.
Bandicam's last versions for Windows XP coincided with the implementation of Nvidia's NVENC system, which they didn't bother to implement in XP. (It would have required extra work on their part to replace DLL functions available by default in Windows 7 and newer.) That is my understanding. Anyway, their last versions do run on XP and do claim to offer NVENC, but the recording won't work of course. They just got messy in their transitional dumping XP phase.
I don't "need" XP to run anything I can think of (although I have noticed running things in Windows 7 various problems I just don't have in XP), I just hate most of the changes and tried as long as I could to avoid the newer OSes.
However, now that I bought this new card and went down this rabbit hole, I tried OBS (which also never ran on XP for reasons). It's, eh, workable, a little odd, OBS abandoned Windows 7 development years ago, so that more or less tells me what kind of people they are. They are not my people. But it's free, it works, and it implements NVENC, which I can affirm produces lossless recordings around 1/6th the size of what x264 made in XP of the same stuff. That is a massive incentive to record only in that environment. Now if only programs all worked correctly and Windows didn't make a mess of files...seriously, why not just let me turn off sorting in a window? Why does a file named "acorn" need to be 50 miles above the other file I want to see called "zebra?"
That said, I strongly value the backwards compatibility of the GeForce 750, whatever it's worth, it's an amazing card that does everything I need. Well aware new software would make my whole system slog, but I rarely get into that.
Thanks for all the suggestions, they were helpful.