I'm not sure which version of the drivers you have, but these work with Poser Pro 2012 on my HD 4250 secondary display, and the 5570 primary display (link is to the 64-bit driver)...

http://support.amd.com/us/gpudownload/windows/legacy/Pages/legacy-radeonaiw-vista64.aspx

EDIT: I saw your OS in the sig; here are the 32-bit Vista drivers...

http://support.amd.com/us/gpudownload/windows/legacy/Pages/legacy-radeonaiw-vista32.aspx

PS; the HD 4850 chipset on the card you are using is about 5 years old...

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