Press CTRL+ALT+DELETE. If it is giving error, it could be a virus. Solutions-
1-Pull down the menu Start, Run, and typegpedit.msc (On some systems (XP home), this program is not installed in this case, download and install it. Open the file, then click on gpedInst.)
http://www.microsoft.com/downloads/details.aspx?familyid=92759d4b-7112-4b6c-ad4a-bbf3802a5c9b&displaylang=enhttp://www.microsoft.com/downloads/details.aspx?familyid=92759d4b-7112-4b6c-ad4a-bbf3802a5c9b&displaylang=en
2-Click User Configuration, Administrative Templates, System, option Ctrl + Alt + Delete
-Right click on Remove Task Manager, then properties.
-By default, everything should not be configured. Select Disable or not configured, and OK
-Restart to save the new settings.
Or Method 2
Click Start, then click Run, type in regedit, and click on OK.
Expand these registry keys in turn:
* HKEY_CURRENT_USER
* Software
* Microsoft
* Windows
* CurrentVersion
* Policies
* System
Right Click on the DisableTaskMgr item. Click on Delete, confirm that you want to delete, and Task Manager should be available once again.