There is a Microsoft Knowledge Base article that addresses this issue, "Task Manager has been disabled by your administrator" error message
Two warnings. If this was not caused by a malware or system error do not modify the computer. If this is a work or school computer you could be fired or ex spelled.
Two, you are going to modify your registry BACK UP YOUR REGISTRY!!!!! I cannot stress how important backing up your registry is. If you make a mistake you can end up reinstalling your operating system.
I use, trust, and recommend ERUNT Registry Backup. It is a great freeware utility that backs up your registry.
I back up my registry once a week. I burn it to a CD. If you use a multi-session CD you can get almost 2 yrs of backup to one CD.
As far as the System Hanging, have you checked the Event Viewer
located in the Administrative Tools
?How to use the Event Viewer