Removing administrator privileges

Here is the problem in a nutshell. I have Vista Home Premium with several users. I have an account with admin privileges, and the other users were standard. The admin account was invisible, but no password (the default I think). One of the standard users must have gotten in to the admin account, and then gave themselves admin privileges. I've tried removing these privileges, but can't. If I try to change to standard user, it keeps coming back as having admin privileges. I tried in safe mode as admin (using netplwiz), but no luck. I can change admin privileges on my own account, or any other account, but not on this one. I even changed the password on the offending account and logged in with that userid, but still can't change admin privileges, even from within the account. Trying to delete the account doesn't work either.

Any suggestions, short of hanging the offending user upside down by their ankles?

Well, maybe the user has used the task scheduler to give him/herself admin privileges on system start up. That's what I would do :P

Go into the taskschd.msc and look for any tasks that involve a "net" command or the users username.

Or if you cant be bothered doing the technical stuff delete the user and make him/her a new account with a different username.