run a full system scan, follow the instructions on the page in my signature
2)login as administrator (not as user with admin rights, but ADMINISTRATOR)
3)use gpedit.msc to set the policy back
Are you previously infected with the Mosucker trojan?
Use the following file to enable Task Manager.
To use the file, rename the file extension to .bat
You could also do it yourself with the instruction in the BAT file.
If none of the above work, you can try this - http://www.mvps.org/sramesh2k/ToolsQuit.htm. Contains a list of all viruses that may be causing problems, although Avast! should pick these up.
Or you can try this: (this option makes more sense to me, since you apparently don’t have a virus):
Perform the following steps to gain access to the group policy settings in Windows XP:
Click Start | Run.
Enter gpedit.msc at the command line and click OK. This will open the Group Policy settings window shown in Figure B.
Select User Configuration | Administrative Templates | System | Ctrl+Alt+Delete Options | Remove Task Manager.
Double-click the Remove Task Manager option from the Group Policy menu. You can then disable, enable, or set the policy to Not Configured. Remember: Since the policy in question is called Remove Task Manager, by disabling the policy, you are actually enabling the Task Manager. Disabling or setting this policy to Not Configured should alleviate Main’s problem.
Just noticed Artras posted, this, I guess this gives you a bit more detail.