I don't know what PPP slot 0 is, but here are some general things you can try: try deleting MacTCP DNR from Sys Folder, MacTCP Prep & TCP/IP Prefs from Preferences folder, and zapping PRAM. other than that I don't know... I assume this is dial-up modem internet access? did you have an account before you upgraded to 9.1?
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be happy!
-mac freak