Access is denied error

Hi , i get an access is denied error when i try to change any thing in msconfig. It’s my own personal pc running windows xp pro.Use simple file sharing is disabled , i’ve checked the mcafee firewall and repaired all registry settings.I’ve added and removed profiles and at one stage gave read and right permissions to all profiles in user accounts and the console.The settings in msconfig change all the time i edit but the error pops up still.I am the administrator and i do not expierience this problem anywhere else.I’ve also ruled out virus and trojan activity.The system is prestine other than this.