I can't delete or reset password on accounts in Snow Leopard

Question

I have this strange situation in Mac OS X (10.6.8) wherein 2 accounts cannot be deleted nor can I reset their passwords.

If I try to reset their passwords I get "Reset Password Failed". I have tried booting with an install CD and using the Password Reset Utility - no joy.

If I try to delete the Account using the minus in Accounts, the Full Name disappears from the icon but the icon remains. If I exit Accounts and come back the Full Name is there again with the icon.

If I (sudo and) look at /Users only one of the accounts shows up.

Other things I have tried include doing the same above (password reset, delete account) as root rather than an Account with Admin privileges, Repairing the Disk (did find some problem with the volume but fixed it) and Repairing the Permissions.

Is the Netinfo database corrupted or something? How do I fix this?