Interesting, the reason I wonder is that I know ISPConfig created entries in /etc/passwd
But of course I forgot to backup that part before reinstalling.
Did you wipe the drive before installing again? or how did you "Uninstall"?
I had a complete Wipe, so only thing I still have are the SQL dumps, the Home directory(Not used by ISPConfig) and the /var/www (Containing webs, and user Maildir)