Propack here. 512 ram and plenty of harddrive space and swapfile.

I loaded a previously saved scene and immediately changed the render options and tried to render to a new window at a modest 750x750.

Well, it happened. The famous 'not enough harddrive space to complete render' or somesuch message when rendering to a new window. And Poser refused to render.

I'm quite sure it has NOTHING to do with memory or harddrive space or swapfile.

I did two things and I don't know which one fixed it.
(1) I moved a prop a bit to one side in the scene (note I didn't delete anything)
(2) I saved the project to a new file

Tried to render again and it worked.

You tell me. But I suspect, and this is weird, that maybe Poser wants you to actually DO something before you render. LOL Haven't had time to test that theory though for whatever it's worth.