Run Diskwarrior prior to re-installing OS 9. Check for bad users, run in safe mode, try to recover Poser 4 folder, etc. If you have a Keyspan USBserver, any 3rd party USB or ethernet devices or any other machines connected, disconnect all of them, and turn off their associated software, if possible. It sounds like something is dumping a kernel extension in there that is screwing up Classic and/or the Poser installer. However, if you backed up Poser 4 from a previous installation, it doesn't need to be re-installed. Just drag the backed-up Poser 4 folder to your clean OS 9 disk (or apps folder).