Some nasty viruses or other malware like to disable task manager. Most of the time, you can repair task manager by editing the registry. Please do not edit the registry unless you know what you are doing! With that warning - to try this fix just follow these steps:

Click on the Windows key and the R key at the same time. A little “Run” dialog box pops up.
Type REGEDIT, hit ENTER. If you’re running Vista, press CONTINUE.
Browse to the key:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\System
In the right pane, look for the value: DisableTaskMgr
Right click DisableTaskMgr and select Delete.
Then browse to the key:
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Policies\System
In the right pane, look for the value: DisableTaskMgr
Right click DisableTaskMgr and select Delete.
Close the Registry by clicking on its big red X.

The change should take effect immediately. If not, reboot and see if Task Manager is now repaired.