Ok, this was reported as problems with installing my code, I have checked the problem, and it exists in the forum code too.
Symptoms.
If you change from DB to NT authorisation, everything is fine, howver if you try to change back, it will not let you, keeps saying you must be the administrator.
Fix.
In admin_config_sytem.asp look for the two lines like this
mLev = cint(ChkUser2(Request.Cookies(strUniqueID & "User")("Name"), Request.Cookies(strUniqueID & "User")("Pword")))
change them to
mLev = cint(ChkUser2(strDBNTUsername, Request.Cookies(strUniqueID & "User")("Pword")))
That fixes the problem, basically chkuser2 was being passed the wrong username.