assistant setup running again and again

Hello all,

I have received from family a max osx 10.5 but we did'nt know the admin password.
So i followed the procedure of remove the flag file (.applesetupdone)
It went right but when setup has finished , the mac continue boot sequence but comes back with the assistant setup again.
The assistznt setup comes back again and again.
it looks like it cannot write the flag back.

Doe anyone has an idea to help.
Nothing found on the web.

Thanks in advance

Copeing

Why not just do a clean install? Start afresh, no hacks, you choose the passwords. Far less hassle and a more stable machine