I have read that changing to 1 will disable Task Manager

Key: Software\Microsoft\Windows\CurrentVersion\Policies\System
Name: DisableTaskMgr
Value: 1=Enablethis key, that is DISABLE TaskManager
Value: 0=Disablethis key, that is Don't Disable, Enable TaskManager

Line 113

On that page, scroll down to line 113 and in the left-hand column, click on "Enable" to download a small .reg file. Save it to the hard drive.

The problem is often caused by malware; either spyware or a virus. Once you have Task Manager running again, you should do complete system scans with your antivirus and some spyware removal tools.

Perhaps you should do a scan if you still can't enable Task Manager.