Sounds like a virtual memory problem indeed.
I'd advise setting the virtual memory min and max at 2 times your physical RAM - WIndows XP Home probably won't let you set it at more than 4092 MB.

You can set your virtual memory by right-clicking the My Computer icon, choose Properties, the Advanced tab. Then click the Settings button under the Performance heading, choose the Advanced tab of the Performance Options dialog that pops up, and click the Change button under the Virtual Memory heading.
In a default Windows install, the virtual memory is probably set to "System managed size". Change that to "Custom size", select the C: drive in the drive list, and set both Initial Size and Maximum Size to 4092. Click the "Set" button to confirn Then "OK" your way out of the dialog boxes.

Chances are that you'll have to reboot your system - you can do so after closing any open programs.

What will this procedure do? You'll get a page file that's large enough to accomodate the memory requests of Poser during rendering.
The problem you experience is not due to a shortage of free memory, it's due to a shortage of contiguous free memory, both in RAM and in your pagefile. A full reboot will clean up both virtual memory and RAM, so memory requests for large blocks can be fulfilled again.
That is why a scene that refused to render before sometimes renders well directly after a reboot.

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