Just wanted to chime in as a WinXP AND UVMapper user. I doubt it has anything to do with WinXP. Mine works fine. It is a known bug that you can't open obj-files with uvmapper by dragging them to the uvmapper ICON OR doubleclicking on an obj if you have associated it with UVMapper. But normal "file - open" from within UVMapper shouldn't give you any problems. At least I've never had any, neither with UVMapper classic nor Pro.

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You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.