Dunno. Could be any of 1001 things. Lack of memory resources, conflict with installed anti-virus, presence of a virus in system, firewall, bad settings in XP, lack of adequate swap file... When such things happen to me, I first verify my computer meets basic program requirements, then I go into XP's Task Manager Applications (accessible thru Ctrl-Alt-Delete) and turn off all apps, background tasks, services, etc that aren't absolutely necessary to keep the computer on, and then see if the program starts. If it does, then it's just tedious trial-and-error to find out what's causing the problem. If not, i try different things until I get some kind of a specific error message. Then I do a "Google" search on the specific message to see what's up. Don't know if this'll help in your case, but that's the process I use. Might also try turning off your video card's acceleration as well, just in case it's a Vid driver issue. BTW, Poser 5 works fine on my XP computer with 1 Gig Ram, a P4 2.8G, and 240 gig of FAT32 HD space. Lots of posts here over the past couple of years by XP users very happy with it's use with Poser 5, so it's likely not XP's fault specifically.