Okay, I had "update on startup" checked, and sometimes I get the error and sometimes I don't. I also have Ethereal, which is a PC-based network analyzer, and I'm here to tell you that this isn't caused by loading V4, or fiddling about with your runtimes, or some mysterious XML virus, or Jupiter aligning with Mars. It's caused by e-frontier's update system being broken. When the updater actually contacts EF, it gets an error message back, and it spits up this XML error. This also appears to be making the update piece of Poser behave peculiarly, so if you have "update on startup" checked, I'd uncheck it if I were you.

I  have to tell you this is not filling me with confidence about being able to download P7 on Monday . . .