Probably your Flash plug-in. Mac had some problems with malware that would take advantage of security weaknesses with the flash plug-in, specifically the FlashBack trojan. IIRC, Poser 8 is the first version of Poser that utilizes the Flash plug-in for the library. Unfortunately for you this is some kind of inadvertant consequence as the FlashBack trojan is specifically known for effecting PPC based Macs and older versions of the flash plug-in, while the OS version you're using is specifically for Intel based Macs. Poser 8 is a universal binary.

Maybe you can update your Flash plug-in? Don't know what other problems this might cause, though.

Edit- Yup, has to be Flash as an .XMP extension is related to Adobe, the makers of Flash.

My advice to you from now on would be to research the potential consequences of any updates that are being force fed to you via Apple, there's a ton of information and forums on the web regarding such things.