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.
Hope this solves the problem. If you are unable to open REGEDIT even and
your computer restart automatically when you type REGEDIT in run then your PC
surely infected with Spyware/Virus or Trojans. Make a full system scan for
viruses and spyware.
Hope this information helps you, let us know!