Hi and welcome to Bleepingcomputer.
Try the following and see if it works:
Go to Start>Run>type in "REG add HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\System /v DisableTaskMgr /t REG_DWORD /d 0 /f"(without the quotes) and press enter, there will be a warning saying something like do you want to add this to the registry please select "yes".
If that doesn't work try the following its the same thing but sometimes other people just get some error saying Windows cannot...
Copy and Paste this on Notepad:
Windows Registry Editor Version 5.00
then go to File>Save as
Save this on your desktop and call it fix.reg under the save as type click "All files"
Now double click on the fix.reg it will say something like do you want to merge the following... please say yes and now try if Taskmanager is wroking now.
Edited by extremeboy, 09 June 2008 - 04:58 PM.