doh !!! When in doubt RTFM
The uninstaller gives you the following options to select from:
Partial Deinstallation: Only the ISPConfig system itself will be uninstalled, but not the
webs, users, DNS records etc. created by it.
Complete Deinstallation: The ISPConfig system and also the objects created by it (webs,
users, DNS records, etc.) will be uninstalled.