If you cannot access the task manager (when you press Ctrl + Alt + Delete, you have the following message: "Task Manager has been disabled by your administrator!"), This trick will allow you to reactivate it. However, it must have an administrator account.
Pull down the menu Start, Run, and type gpedit.msc
On some systems (XP home), this program is not installed in this case, download and install it. Open the file, then click on gpedInst.
Click User Configuration, Administrative Templates, System, option Ctrl + Alt + Delete
Right click on Remove Task Manager, then properties.
By default, everything should not be configured. Select Disable or not configured, and OK
Restart to save the new settings.
This trick is also disable other applications, such as System Restore, Control Panel, Add and remove programs, and many more!
Disabling the task manager - and other applications - can be the cause of a mistake, but often it is an infection (virus): For sure, scan your system!
Another method you can use, is using a REG fix download here
Run this fix and it will solve the problem automatically!