See if this could help
http://windowsxp.mvps.org/Taskmanager_error.htm

and

http://www.pchell.com/support/taskmanagerdisabled.shtml

>Hi all
>
>I had to re-format my HDD on Saturday. Now I am not able to access the Task Manager, getting the message "Task Manager has been disabled by your administrator.", well, I am the Administrator and have confirmed the user rights.
>
>Something else, Skype is hogging CPU/Memory, can't confirm which as I do not have Task Manager access.
>
>Same hardware, softwares and operating system yet what changed?