Wombat, I know you are experienced enough not to lightly point the finger at SR4 - but as a programmer, I am skeptical that the SR4 updater caused the problems you encountered, especially after uli's description of what SR4 Installer does. I would suspect that your registry had already gotten corrupted somehow, and that this installation was the last straw that pushed Windows over the edge. But XP unable to recover? If it were my cheap PC clone, I'd start suspecting hard drive, power supply, or motherboard were no longer 100% reliable. For anyone who wants to take no chances, install or upgrade Poser 5 (or any other complex multimedia application) FIRST THING after booting Windows. Ideally without virus checker, Windows Messenger, or any of those other background processes: Start Menu / "Run..." button. Type "msconfig". Click OK. Click "Startup" tab. Check "Disable All". (You DON'T want to disable stuff under any other tab, unless you REALLY know what you are doing.) Reboot. Install Poser 5. Run "msconfig" again. Check "Normal Startup". Reboot. Personal Note: I myself never bother to do this, when running Windows XP or Windows 2000. Older OS versions were MUCH less reliable. But then, I also format my boot drives, and install everything from scratch, so that I know exactly what is running on each PC...