|If you have been infected by a virus in the past, that virus may have disabled your task manager through the registry.|
this was copied from http://windows.microsoft.com/en-US/...
(please use extreme caution in changing the registry)
Open the Registry Editor by clicking the Start button , typing regedit into the Search box, and then pressing ENTER. If you are prompted for an administrator password or confirmation, type the password or provide confirmation.
Navigate to the following location: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies
Double-click System, double-click DisableTaskMgr, change Value data to 0, and then click OK.