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if it still runs slow, maybe you changed some video settings? 

You mean video card settings? If that's what you mean, the video card contributes absolutely nothing - zero, zip, zilch, nada - to the render. It's all in the CPU for rendering, and somewhat in the RAM.

The fact ladydeuce mentions Poser files becoming corrupted as well as display doesn't sound good at all. Could be just a faulty Poser installation, configuration files gone bad, or it could be the first signs of failing RAM or a disk with corrupted data or bad sectors. Or overheating CPU, RAM, or disk.

I would first uninstall Poser then run defrag, then reinstall Poser and see if the problem is still there.
Might want to uninstall/reinstall the video drivers too, just in case that's part of the display problem.

If the problems are still there I would then run disk check, and probably the freeware MemTest if I were you. If this was all working fine before but not now, it doesn't sound good at all.
I would also have a good look at the temperatures that are being created by the RAM and CPU during a render, to see if that's a problem. A CPU will cycle itself back when it starts overheating. At a certain point it will actually simply shut down your computer (in theory) before it gets damaged.
An overheating GPU could be the source of the display issue, too.

Freeware temperature monitoring software:
www.cpuid.com/hwmonitor.php

Assuming no problems with that, but problems still with Poser, I'd probably reinstall Windows and then reinstall everything else.

MemTest:
www.memtest.org/

You didn't mention how much RAM you have. If Poser is using your swap file for RAM, and your disk is going bad, that could be a reason for suddenly slower renders. Same deal with RAM going bad or overheating. But the file corruption is a bad sign, especially when it's suddenly happening at the same time your renders are slowing down.

Of course since everything else is running OK, that's a good sign it may simply be a Poser install gone bad, but you never know, and it's worth it to take a little time to check out your whole system.