Try this:
Maybe, your computer is infected with Spyware or Virus. You can try the
following to resolve the problem:
For Windows XP Professional
===================
1. Click Start - Run - type GPEDIT.MSC
2. Expand To:
User Configuration
Administrative Templates
System
CTRL+ALT+Del Option
3. On the right-side pane check for an entry called "Remove Task Manager".
4. If the "State" is set to "Not Configured" or "Enabled" then double-click
the entry and set it to "Disable".
Check if you can now open the Task Manager or make a Restart and then try it.
If still that doesn't work then: (For both Windows XP Professional and Home
Edition)
1. Click Start - Run - type REGEDIT and press Enter key.
2. Expand To:
HKEY_CURRENT_USER
Software
Microsoft
Windows
CurrentVersion
Policies
System
3. In the Right-side pane check for an entry called "DisableTaskMgr".
4. If it is there, select the entry and delete it.
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Hope this information helps you, let us know!