Task manager can stop working for a number of reasons, I'll go over some fixes from easiest to hardest
See if taskmgr exists
1) Click Start -> Search
2) Click All files and folders
3) Search for taskmgr and set location to %systemroot% (usually C:\Windows or C:\winnt). You can actually type "%systemroot%" as well.
4) You should find in the list:
If you cannot find this file, you can extract it off your XP CD again. Refer to APPENDIX on how to extract the file. The help files CHM and HLP are not needed to run taskmgr.
Once the files are in place, try running taskmgr again. Try double-clicking on the program icon itself in the system32 folder.
If you can run Task Manager by double-clicking it, but NOT by ctrl-alt-del or ctrl-shift-esc OR start -> Run Taskmgr, then check the PATH variable:
1) Right Click My Computer and choose Properties
2) Click the Advanced tab
3) Click the Environment Variables button
4) In the System variables section, find the "Path" entry.
5) Make sure that the following are in this list. Each path is sepparated by a semicolon.
These are EXACT terms, %systemroot% means %systemroot%
Check environment variable systemroot
If the previous PATH entries ARE there, make sure that the %systemroot% variable ITSELF is in tact. Here's how:
1) Click Start -> Run and type %systemroot% and hit Enter.
This should open your Windows folder. Which is almost always C:\Windows, or C:\winnt.
If it doesn't open your Windows folder, but goes somewhere else, or throws an error, refer to Appendix "Fix systemroot". If it DOES open the correct folder, then you're fine on this point.
Once you KNOW that all the right files are there, and the variables are in tact. Move on to...
Check user and system policies
Note that if you have a policy problem, you will usually get an error when you try to open it, such as "The system administrator has disabled Task Manager" or something similar.
1) Make sure you are logged in as a user with administrative privileges. You can see which users are admins by going in Control Panel and clicking the Users icon.
2) Start -> Run regedit
3) Browse to:
4) Click on each subfolder under Policies (it is usually System that has the entry).
5) You will see in the right-hand side, and entry called DisableTaskMgr.
If you see this key (right side), just highlight it and delete the key. Note that if the key's value was "1", that is why you can't open Task Manager.
6) Also check this key:
Repeat from above and delete the key DisableTaskMgr.
If for some reason that doesn't work, I found one case where just typing this into the Run box does the trick, here is the command:
REG add HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\System /v DisableTaskMgr /t REG_DWORD /d 0 /f
It basically does the same thing of changing it manually in the registry. But just in case...
You'll have to log off and on for these changes to take effect.
If Task Manager still doesn't work, check the same registry keys you changed above, if they are back to the way they were before, then you have malware setting it this way. And so...
Clean system of malway and viruses